Merge branch 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull RCU updates from Ingo Molnar:
 "The main changes in this cycle were:

   - Make kfree_rcu() use kfree_bulk() for added performance

   - RCU updates

   - Callback-overload handling updates

   - Tasks-RCU KCSAN and sparse updates

   - Locking torture test and RCU torture test updates

   - Documentation updates

   - Miscellaneous fixes"

* 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (74 commits)
  rcu: Make rcu_barrier() account for offline no-CBs CPUs
  rcu: Mark rcu_state.gp_seq to detect concurrent writes
  Documentation/memory-barriers: Fix typos
  doc: Add rcutorture scripting to torture.txt
  doc/RCU/rcu: Use https instead of http if possible
  doc/RCU/rcu: Use absolute paths for non-rst files
  doc/RCU/rcu: Use ':ref:' for links to other docs
  doc/RCU/listRCU: Update example function name
  doc/RCU/listRCU: Fix typos in a example code snippets
  doc/RCU/Design: Remove remaining HTML tags in ReST files
  doc: Add some more RCU list patterns in the kernel
  rcutorture: Set KCSAN Kconfig options to detect more data races
  rcutorture: Manually clean up after rcu_barrier() failure
  rcutorture: Make rcu_torture_barrier_cbs() post from corresponding CPU
  rcuperf: Measure memory footprint during kfree_rcu() test
  rcutorture: Annotation lockless accesses to rcu_torture_current
  rcutorture: Add READ_ONCE() to rcu_torture_count and rcu_torture_batch
  rcutorture: Fix stray access to rcu_fwd_cb_nodelay
  rcutorture: Fix rcu_torture_one_read()/rcu_torture_writer() data race
  rcutorture: Make kvm-find-errors.sh abort on bad directory
  ...

Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst

@@ -4,7 +4,7 @@ A Tour Through TREE_RCU's Grace-Period Memory Ordering
 
 August 8, 2017
 
-This article was contributed by Paul E. McKenney
+This article was contributed by Paul E. McKenney
 
 Introduction
 ============
@@ -48,7 +48,7 @@ Tree RCU Grace Period Memory Ordering Building Blocks
 
 The workhorse for RCU's grace-period memory ordering is the
 critical section for the ``rcu_node`` structure's
-``->lock``. These critical sections use helper functions for lock
+``->lock``. These critical sections use helper functions for lock
 acquisition, including ``raw_spin_lock_rcu_node()``,
 ``raw_spin_lock_irq_rcu_node()``, and ``raw_spin_lock_irqsave_rcu_node()``.
 Their lock-release counterparts are ``raw_spin_unlock_rcu_node()``,
@@ -102,9 +102,9 @@ lock-acquisition and lock-release functions::
    23   r3 = READ_ONCE(x);
    24 }
    25
-   26 WARN_ON(r1 == 0 && r2 == 0 && r3 == 0);
+   26 WARN_ON(r1 == 0 && r2 == 0 && r3 == 0);
 
-The ``WARN_ON()`` is evaluated at “the end of time”,
+The ``WARN_ON()`` is evaluated at "the end of time",
 after all changes have propagated throughout the system.
 Without the ``smp_mb__after_unlock_lock()`` provided by the
 acquisition functions, this ``WARN_ON()`` could trigger, for example

Documentation/RCU/listRCU.rst

@@ -4,12 +4,61 @@ Using RCU to Protect Read-Mostly Linked Lists
 =============================================
 
 One of the best applications of RCU is to protect read-mostly linked lists
-("struct list_head" in list.h).  One big advantage of this approach
+(``struct list_head`` in list.h).  One big advantage of this approach
 is that all of the required memory barriers are included for you in
 the list macros.  This document describes several applications of RCU,
 with the best fits first.
 
-Example 1: Read-Side Action Taken Outside of Lock, No In-Place Updates
+
+Example 1: Read-mostly list: Deferred Destruction
+-------------------------------------------------
+
+A widely used usecase for RCU lists in the kernel is lockless iteration over
+all processes in the system. ``task_struct::tasks`` represents the list node that
+links all the processes. The list can be traversed in parallel to any list
+additions or removals.
+
+The traversal of the list is done using ``for_each_process()`` which is defined
+by the 2 macros::
+
+	#define next_task(p) \
+		list_entry_rcu((p)->tasks.next, struct task_struct, tasks)
+
+	#define for_each_process(p) \
+		for (p = &init_task ; (p = next_task(p)) != &init_task ; )
+
+The code traversing the list of all processes typically looks like::
+
+	rcu_read_lock();
+	for_each_process(p) {
+		/* Do something with p */
+	}
+	rcu_read_unlock();
+
+The simplified code for removing a process from a task list is::
+
+	void release_task(struct task_struct *p)
+	{
+		write_lock(&tasklist_lock);
+		list_del_rcu(&p->tasks);
+		write_unlock(&tasklist_lock);
+		call_rcu(&p->rcu, delayed_put_task_struct);
+	}
+
+When a process exits, ``release_task()`` calls ``list_del_rcu(&p->tasks)`` under
+``tasklist_lock`` writer lock protection, to remove the task from the list of
+all tasks. The ``tasklist_lock`` prevents concurrent list additions/removals
+from corrupting the list. Readers using ``for_each_process()`` are not protected
+with the ``tasklist_lock``. To prevent readers from noticing changes in the list
+pointers, the ``task_struct`` object is freed only after one or more grace
+periods elapse (with the help of call_rcu()). This deferring of destruction
+ensures that any readers traversing the list will see valid ``p->tasks.next``
+pointers and deletion/freeing can happen in parallel with traversal of the list.
+This pattern is also called an **existence lock**, since RCU pins the object in
+memory until all existing readers finish.
+
+
+Example 2: Read-Side Action Taken Outside of Lock: No In-Place Updates
 ----------------------------------------------------------------------
 
 The best applications are cases where, if reader-writer locking were
@@ -26,7 +75,7 @@ added or deleted, rather than being modified in place.
 
 A straightforward example of this use of RCU may be found in the
 system-call auditing support.  For example, a reader-writer locked
-implementation of audit_filter_task() might be as follows::
+implementation of ``audit_filter_task()`` might be as follows::
 
 	static enum audit_state audit_filter_task(struct task_struct *tsk)
 	{
@@ -34,7 +83,7 @@ implementation of audit_filter_task() might be as follows::
 		enum audit_state   state;
 
 		read_lock(&auditsc_lock);
-		/* Note: audit_netlink_sem held by caller. */
+		/* Note: audit_filter_mutex held by caller. */
 		list_for_each_entry(e, &audit_tsklist, list) {
 			if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
 				read_unlock(&auditsc_lock);
@@ -58,7 +107,7 @@ This means that RCU can be easily applied to the read side, as follows::
 		enum audit_state   state;
 
 		rcu_read_lock();
-		/* Note: audit_netlink_sem held by caller. */
+		/* Note: audit_filter_mutex held by caller. */
 		list_for_each_entry_rcu(e, &audit_tsklist, list) {
 			if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
 				rcu_read_unlock();
@@ -69,18 +118,18 @@ This means that RCU can be easily applied to the read side, as follows::
 		return AUDIT_BUILD_CONTEXT;
 	}
 
-The read_lock() and read_unlock() calls have become rcu_read_lock()
+The ``read_lock()`` and ``read_unlock()`` calls have become rcu_read_lock()
 and rcu_read_unlock(), respectively, and the list_for_each_entry() has
-become list_for_each_entry_rcu().  The _rcu() list-traversal primitives
+become list_for_each_entry_rcu().  The **_rcu()** list-traversal primitives
 insert the read-side memory barriers that are required on DEC Alpha CPUs.
 
-The changes to the update side are also straightforward.  A reader-writer
-lock might be used as follows for deletion and insertion::
+The changes to the update side are also straightforward. A reader-writer lock
+might be used as follows for deletion and insertion::
 
 	static inline int audit_del_rule(struct audit_rule *rule,
 					 struct list_head *list)
 	{
-		struct audit_entry  *e;
+		struct audit_entry *e;
 
 		write_lock(&auditsc_lock);
 		list_for_each_entry(e, list, list) {
@@ -113,9 +162,9 @@ Following are the RCU equivalents for these two functions::
 	static inline int audit_del_rule(struct audit_rule *rule,
 					 struct list_head *list)
 	{
-		struct audit_entry  *e;
+		struct audit_entry *e;
 
-		/* Do not use the _rcu iterator here, since this is the only
+		/* No need to use the _rcu iterator here, since this is the only
 		 * deletion routine. */
 		list_for_each_entry(e, list, list) {
 			if (!audit_compare_rule(rule, &e->rule)) {
@@ -139,45 +188,45 @@ Following are the RCU equivalents for these two functions::
 		return 0;
 	}
 
-Normally, the write_lock() and write_unlock() would be replaced by
-a spin_lock() and a spin_unlock(), but in this case, all callers hold
-audit_netlink_sem, so no additional locking is required.  The auditsc_lock
-can therefore be eliminated, since use of RCU eliminates the need for
-writers to exclude readers.  Normally, the write_lock() calls would
-be converted into spin_lock() calls.
+Normally, the ``write_lock()`` and ``write_unlock()`` would be replaced by a
+spin_lock() and a spin_unlock(). But in this case, all callers hold
+``audit_filter_mutex``, so no additional locking is required. The
+``auditsc_lock`` can therefore be eliminated, since use of RCU eliminates the
+need for writers to exclude readers.
 
 The list_del(), list_add(), and list_add_tail() primitives have been
 replaced by list_del_rcu(), list_add_rcu(), and list_add_tail_rcu().
-The _rcu() list-manipulation primitives add memory barriers that are
-needed on weakly ordered CPUs (most of them!).  The list_del_rcu()
-primitive omits the pointer poisoning debug-assist code that would
-otherwise cause concurrent readers to fail spectacularly.
+The **_rcu()** list-manipulation primitives add memory barriers that are needed on
+weakly ordered CPUs (most of them!).  The list_del_rcu() primitive omits the
+pointer poisoning debug-assist code that would otherwise cause concurrent
+readers to fail spectacularly.
 
-So, when readers can tolerate stale data and when entries are either added
-or deleted, without in-place modification, it is very easy to use RCU!
+So, when readers can tolerate stale data and when entries are either added or
+deleted, without in-place modification, it is very easy to use RCU!
 
-Example 2: Handling In-Place Updates
+
+Example 3: Handling In-Place Updates
 ------------------------------------
 
-The system-call auditing code does not update auditing rules in place.
-However, if it did, reader-writer-locked code to do so might look as
-follows (presumably, the field_count is only permitted to decrease,
-otherwise, the added fields would need to be filled in)::
+The system-call auditing code does not update auditing rules in place.  However,
+if it did, the reader-writer-locked code to do so might look as follows
+(assuming only ``field_count`` is updated, otherwise, the added fields would
+need to be filled in)::
 
 	static inline int audit_upd_rule(struct audit_rule *rule,
 					 struct list_head *list,
 					 __u32 newaction,
 					 __u32 newfield_count)
 	{
-		struct audit_entry  *e;
-		struct audit_newentry *ne;
+		struct audit_entry *e;
+		struct audit_entry *ne;
 
 		write_lock(&auditsc_lock);
-		/* Note: audit_netlink_sem held by caller. */
+		/* Note: audit_filter_mutex held by caller. */
 		list_for_each_entry(e, list, list) {
 			if (!audit_compare_rule(rule, &e->rule)) {
 				e->rule.action = newaction;
-				e->rule.file_count = newfield_count;
+				e->rule.field_count = newfield_count;
 				write_unlock(&auditsc_lock);
 				return 0;
 			}
@@ -188,16 +237,16 @@ otherwise, the added fields would need to be filled in)::
 
 The RCU version creates a copy, updates the copy, then replaces the old
 entry with the newly updated entry.  This sequence of actions, allowing
-concurrent reads while doing a copy to perform an update, is what gives
-RCU ("read-copy update") its name.  The RCU code is as follows::
+concurrent reads while making a copy to perform an update, is what gives
+RCU (*read-copy update*) its name.  The RCU code is as follows::
 
 	static inline int audit_upd_rule(struct audit_rule *rule,
 					 struct list_head *list,
 					 __u32 newaction,
 					 __u32 newfield_count)
 	{
-		struct audit_entry  *e;
-		struct audit_newentry *ne;
+		struct audit_entry *e;
+		struct audit_entry *ne;
 
 		list_for_each_entry(e, list, list) {
 			if (!audit_compare_rule(rule, &e->rule)) {
@@ -206,7 +255,7 @@ RCU ("read-copy update") its name.  The RCU code is as follows::
 					return -ENOMEM;
 				audit_copy_rule(&ne->rule, &e->rule);
 				ne->rule.action = newaction;
-				ne->rule.file_count = newfield_count;
+				ne->rule.field_count = newfield_count;
 				list_replace_rcu(&e->list, &ne->list);
 				call_rcu(&e->rcu, audit_free_rule);
 				return 0;
@@ -215,34 +264,45 @@ RCU ("read-copy update") its name.  The RCU code is as follows::
 		return -EFAULT;		/* No matching rule */
 	}
 
-Again, this assumes that the caller holds audit_netlink_sem.  Normally,
-the reader-writer lock would become a spinlock in this sort of code.
+Again, this assumes that the caller holds ``audit_filter_mutex``.  Normally, the
+writer lock would become a spinlock in this sort of code.
 
-Example 3: Eliminating Stale Data
+Another use of this pattern can be found in the openswitch driver's *connection
+tracking table* code in ``ct_limit_set()``.  The table holds connection tracking
+entries and has a limit on the maximum entries.  There is one such table
+per-zone and hence one *limit* per zone.  The zones are mapped to their limits
+through a hashtable using an RCU-managed hlist for the hash chains. When a new
+limit is set, a new limit object is allocated and ``ct_limit_set()`` is called
+to replace the old limit object with the new one using list_replace_rcu().
+The old limit object is then freed after a grace period using kfree_rcu().
+
+
+Example 4: Eliminating Stale Data
 ---------------------------------
 
-The auditing examples above tolerate stale data, as do most algorithms
+The auditing example above tolerates stale data, as do most algorithms
 that are tracking external state.  Because there is a delay from the
 time the external state changes before Linux becomes aware of the change,
-additional RCU-induced staleness is normally not a problem.
+additional RCU-induced staleness is generally not a problem.
 
 However, there are many examples where stale data cannot be tolerated.
-One example in the Linux kernel is the System V IPC (see the ipc_lock()
-function in ipc/util.c).  This code checks a "deleted" flag under a
-per-entry spinlock, and, if the "deleted" flag is set, pretends that the
+One example in the Linux kernel is the System V IPC (see the shm_lock()
+function in ipc/shm.c).  This code checks a *deleted* flag under a
+per-entry spinlock, and, if the *deleted* flag is set, pretends that the
 entry does not exist.  For this to be helpful, the search function must
-return holding the per-entry spinlock, as ipc_lock() does in fact do.
+return holding the per-entry spinlock, as shm_lock() does in fact do.
+
+.. _quick_quiz:
 
 Quick Quiz:
-	Why does the search function need to return holding the per-entry lock for
-	this deleted-flag technique to be helpful?
+	For the deleted-flag technique to be helpful, why is it necessary
+	to hold the per-entry lock while returning from the search function?
 
-:ref:`Answer to Quick Quiz <answer_quick_quiz_list>`
+:ref:`Answer to Quick Quiz <quick_quiz_answer>`
 
-If the system-call audit module were to ever need to reject stale data,
-one way to accomplish this would be to add a "deleted" flag and a "lock"
-spinlock to the audit_entry structure, and modify audit_filter_task()
-as follows::
+If the system-call audit module were to ever need to reject stale data, one way
+to accomplish this would be to add a ``deleted`` flag and a ``lock`` spinlock to the
+audit_entry structure, and modify ``audit_filter_task()`` as follows::
 
 	static enum audit_state audit_filter_task(struct task_struct *tsk)
 	{
@@ -267,20 +327,20 @@ as follows::
 	}
 
 Note that this example assumes that entries are only added and deleted.
-Additional mechanism is required to deal correctly with the
-update-in-place performed by audit_upd_rule().  For one thing,
-audit_upd_rule() would need additional memory barriers to ensure
-that the list_add_rcu() was really executed before the list_del_rcu().
+Additional mechanism is required to deal correctly with the update-in-place
+performed by ``audit_upd_rule()``.  For one thing, ``audit_upd_rule()`` would
+need additional memory barriers to ensure that the list_add_rcu() was really
+executed before the list_del_rcu().
 
-The audit_del_rule() function would need to set the "deleted"
-flag under the spinlock as follows::
+The ``audit_del_rule()`` function would need to set the ``deleted`` flag under the
+spinlock as follows::
 
 	static inline int audit_del_rule(struct audit_rule *rule,
 					 struct list_head *list)
 	{
-		struct audit_entry  *e;
+		struct audit_entry *e;
 
-		/* Do not need to use the _rcu iterator here, since this
+		/* No need to use the _rcu iterator here, since this
 		 * is the only deletion routine. */
 		list_for_each_entry(e, list, list) {
 			if (!audit_compare_rule(rule, &e->rule)) {
@@ -295,6 +355,91 @@ flag under the spinlock as follows::
 		return -EFAULT;		/* No matching rule */
 	}
 
+This too assumes that the caller holds ``audit_filter_mutex``.
+
+
+Example 5: Skipping Stale Objects
+---------------------------------
+
+For some usecases, reader performance can be improved by skipping stale objects
+during read-side list traversal if the object in concern is pending destruction
+after one or more grace periods. One such example can be found in the timerfd
+subsystem. When a ``CLOCK_REALTIME`` clock is reprogrammed - for example due to
+setting of the system time, then all programmed timerfds that depend on this
+clock get triggered and processes waiting on them to expire are woken up in
+advance of their scheduled expiry. To facilitate this, all such timers are added
+to an RCU-managed ``cancel_list`` when they are setup in
+``timerfd_setup_cancel()``::
+
+	static void timerfd_setup_cancel(struct timerfd_ctx *ctx, int flags)
+	{
+		spin_lock(&ctx->cancel_lock);
+		if ((ctx->clockid == CLOCK_REALTIME &&
+		    (flags & TFD_TIMER_ABSTIME) && (flags & TFD_TIMER_CANCEL_ON_SET)) {
+			if (!ctx->might_cancel) {
+				ctx->might_cancel = true;
+				spin_lock(&cancel_lock);
+				list_add_rcu(&ctx->clist, &cancel_list);
+				spin_unlock(&cancel_lock);
+			}
+		}
+		spin_unlock(&ctx->cancel_lock);
+	}
+
+When a timerfd is freed (fd is closed), then the ``might_cancel`` flag of the
+timerfd object is cleared, the object removed from the ``cancel_list`` and
+destroyed::
+
+	int timerfd_release(struct inode *inode, struct file *file)
+	{
+		struct timerfd_ctx *ctx = file->private_data;
+
+		spin_lock(&ctx->cancel_lock);
+		if (ctx->might_cancel) {
+			ctx->might_cancel = false;
+			spin_lock(&cancel_lock);
+			list_del_rcu(&ctx->clist);
+			spin_unlock(&cancel_lock);
+		}
+		spin_unlock(&ctx->cancel_lock);
+
+		hrtimer_cancel(&ctx->t.tmr);
+		kfree_rcu(ctx, rcu);
+		return 0;
+	}
+
+If the ``CLOCK_REALTIME`` clock is set, for example by a time server, the
+hrtimer framework calls ``timerfd_clock_was_set()`` which walks the
+``cancel_list`` and wakes up processes waiting on the timerfd. While iterating
+the ``cancel_list``, the ``might_cancel`` flag is consulted to skip stale
+objects::
+
+	void timerfd_clock_was_set(void)
+	{
+		struct timerfd_ctx *ctx;
+		unsigned long flags;
+
+		rcu_read_lock();
+		list_for_each_entry_rcu(ctx, &cancel_list, clist) {
+			if (!ctx->might_cancel)
+				continue;
+			spin_lock_irqsave(&ctx->wqh.lock, flags);
+			if (ctx->moffs != ktime_mono_to_real(0)) {
+				ctx->moffs = KTIME_MAX;
+				ctx->ticks++;
+				wake_up_locked_poll(&ctx->wqh, EPOLLIN);
+			}
+			spin_unlock_irqrestore(&ctx->wqh.lock, flags);
+		}
+		rcu_read_unlock();
+	}
+
+The key point here is, because RCU-traversal of the ``cancel_list`` happens
+while objects are being added and removed to the list, sometimes the traversal
+can step on an object that has been removed from the list. In this example, it
+is seen that it is better to skip such objects using a flag.
+
+
 Summary
 -------
 
@@ -303,19 +448,21 @@ the most amenable to use of RCU.  The simplest case is where entries are
 either added or deleted from the data structure (or atomically modified
 in place), but non-atomic in-place modifications can be handled by making
 a copy, updating the copy, then replacing the original with the copy.
-If stale data cannot be tolerated, then a "deleted" flag may be used
+If stale data cannot be tolerated, then a *deleted* flag may be used
 in conjunction with a per-entry spinlock in order to allow the search
 function to reject newly deleted data.
 
-.. _answer_quick_quiz_list:
+.. _quick_quiz_answer:
 
 Answer to Quick Quiz:
-	Why does the search function need to return holding the per-entry
-	lock for this deleted-flag technique to be helpful?
+	For the deleted-flag technique to be helpful, why is it necessary
+	to hold the per-entry lock while returning from the search function?
 
 	If the search function drops the per-entry lock before returning,
 	then the caller will be processing stale data in any case.  If it
 	is really OK to be processing stale data, then you don't need a
-	"deleted" flag.  If processing stale data really is a problem,
+	*deleted* flag.  If processing stale data really is a problem,
 	then you need to hold the per-entry lock across all of the code
 	that uses the value that was returned.
+
+:ref:`Back to Quick Quiz <quick_quiz>`

Documentation/RCU/rcu.rst

@@ -11,8 +11,8 @@ must be long enough that any readers accessing the item being deleted have
 since dropped their references.  For example, an RCU-protected deletion
 from a linked list would first remove the item from the list, wait for
 a grace period to elapse, then free the element.  See the
-Documentation/RCU/listRCU.rst file for more information on using RCU with
-linked lists.
+:ref:`Documentation/RCU/listRCU.rst <list_rcu_doc>` for more information on
+using RCU with linked lists.
 
 Frequently Asked Questions
 --------------------------
@@ -50,7 +50,7 @@ Frequently Asked Questions
 - If I am running on a uniprocessor kernel, which can only do one
   thing at a time, why should I wait for a grace period?
 
-  See the Documentation/RCU/UP.rst file for more information.
+  See :ref:`Documentation/RCU/UP.rst <up_doc>` for more information.
 
 - How can I see where RCU is currently used in the Linux kernel?
 
@@ -68,18 +68,18 @@ Frequently Asked Questions
 
 - Why the name "RCU"?
 
-  "RCU" stands for "read-copy update".  The file Documentation/RCU/listRCU.rst
-  has more information on where this name came from, search for
-  "read-copy update" to find it.
+  "RCU" stands for "read-copy update".
+  :ref:`Documentation/RCU/listRCU.rst <list_rcu_doc>` has more information on where
+  this name came from, search for "read-copy update" to find it.
 
 - I hear that RCU is patented?  What is with that?
 
   Yes, it is.  There are several known patents related to RCU,
-  search for the string "Patent" in RTFP.txt to find them.
+  search for the string "Patent" in Documentation/RCU/RTFP.txt to find them.
   Of these, one was allowed to lapse by the assignee, and the
   others have been contributed to the Linux kernel under GPL.
   There are now also LGPL implementations of user-level RCU
-  available (http://liburcu.org/).
+  available (https://liburcu.org/).
 
 - I hear that RCU needs work in order to support realtime kernels?
 
@@ -88,5 +88,5 @@ Frequently Asked Questions
 
 - Where can I find more information on RCU?
 
-  See the RTFP.txt file in this directory.
+  See the Documentation/RCU/RTFP.txt file.
   Or point your browser at (http://www.rdrop.com/users/paulmck/RCU/).

Documentation/RCU/torture.txt

@@ -124,9 +124,14 @@ using a dynamically allocated srcu_struct (hence "srcud-" rather than
 debugging.  The final "T" entry contains the totals of the counters.
 
 
-USAGE
+USAGE ON SPECIFIC KERNEL BUILDS
 
-The following script may be used to torture RCU:
+It is sometimes desirable to torture RCU on a specific kernel build,
+for example, when preparing to put that kernel build into production.
+In that case, the kernel should be built with CONFIG_RCU_TORTURE_TEST=m
+so that the test can be started using modprobe and terminated using rmmod.
+
+For example, the following script may be used to torture RCU:
 
 	#!/bin/sh
 
@@ -142,8 +147,136 @@ checked for such errors.  The "rmmod" command forces a "SUCCESS",
 two are self-explanatory, while the last indicates that while there
 were no RCU failures, CPU-hotplug problems were detected.
 
-However, the tools/testing/selftests/rcutorture/bin/kvm.sh script
-provides better automation, including automatic failure analysis.
-It assumes a qemu/kvm-enabled platform, and runs guest OSes out of initrd.
-See tools/testing/selftests/rcutorture/doc/initrd.txt for instructions
-on setting up such an initrd.
+
+USAGE ON MAINLINE KERNELS
+
+When using rcutorture to test changes to RCU itself, it is often
+necessary to build a number of kernels in order to test that change
+across a broad range of combinations of the relevant Kconfig options
+and of the relevant kernel boot parameters.  In this situation, use
+of modprobe and rmmod can be quite time-consuming and error-prone.
+
+Therefore, the tools/testing/selftests/rcutorture/bin/kvm.sh
+script is available for mainline testing for x86, arm64, and
+powerpc.  By default, it will run the series of tests specified by
+tools/testing/selftests/rcutorture/configs/rcu/CFLIST, with each test
+running for 30 minutes within a guest OS using a minimal userspace
+supplied by an automatically generated initrd.  After the tests are
+complete, the resulting build products and console output are analyzed
+for errors and the results of the runs are summarized.
+
+On larger systems, rcutorture testing can be accelerated by passing the
+--cpus argument to kvm.sh.  For example, on a 64-CPU system, "--cpus 43"
+would use up to 43 CPUs to run tests concurrently, which as of v5.4 would
+complete all the scenarios in two batches, reducing the time to complete
+from about eight hours to about one hour (not counting the time to build
+the sixteen kernels).  The "--dryrun sched" argument will not run tests,
+but rather tell you how the tests would be scheduled into batches.  This
+can be useful when working out how many CPUs to specify in the --cpus
+argument.
+
+Not all changes require that all scenarios be run.  For example, a change
+to Tree SRCU might run only the SRCU-N and SRCU-P scenarios using the
+--configs argument to kvm.sh as follows:  "--configs 'SRCU-N SRCU-P'".
+Large systems can run multiple copies of of the full set of scenarios,
+for example, a system with 448 hardware threads can run five instances
+of the full set concurrently.  To make this happen:
+
+	kvm.sh --cpus 448 --configs '5*CFLIST'
+
+Alternatively, such a system can run 56 concurrent instances of a single
+eight-CPU scenario:
+
+	kvm.sh --cpus 448 --configs '56*TREE04'
+
+Or 28 concurrent instances of each of two eight-CPU scenarios:
+
+	kvm.sh --cpus 448 --configs '28*TREE03 28*TREE04'
+
+Of course, each concurrent instance will use memory, which can be
+limited using the --memory argument, which defaults to 512M.  Small
+values for memory may require disabling the callback-flooding tests
+using the --bootargs parameter discussed below.
+
+Sometimes additional debugging is useful, and in such cases the --kconfig
+parameter to kvm.sh may be used, for example, "--kconfig 'CONFIG_KASAN=y'".
+
+Kernel boot arguments can also be supplied, for example, to control
+rcutorture's module parameters.  For example, to test a change to RCU's
+CPU stall-warning code, use "--bootargs 'rcutorture.stall_cpu=30'".
+This will of course result in the scripting reporting a failure, namely
+the resuling RCU CPU stall warning.  As noted above, reducing memory may
+require disabling rcutorture's callback-flooding tests:
+
+	kvm.sh --cpus 448 --configs '56*TREE04' --memory 128M \
+		--bootargs 'rcutorture.fwd_progress=0'
+
+Sometimes all that is needed is a full set of kernel builds.  This is
+what the --buildonly argument does.
+
+Finally, the --trust-make argument allows each kernel build to reuse what
+it can from the previous kernel build.
+
+There are additional more arcane arguments that are documented in the
+source code of the kvm.sh script.
+
+If a run contains failures, the number of buildtime and runtime failures
+is listed at the end of the kvm.sh output, which you really should redirect
+to a file.  The build products and console output of each run is kept in
+tools/testing/selftests/rcutorture/res in timestamped directories.  A
+given directory can be supplied to kvm-find-errors.sh in order to have
+it cycle you through summaries of errors and full error logs.  For example:
+
+	tools/testing/selftests/rcutorture/bin/kvm-find-errors.sh \
+		tools/testing/selftests/rcutorture/res/2020.01.20-15.54.23
+
+However, it is often more convenient to access the files directly.
+Files pertaining to all scenarios in a run reside in the top-level
+directory (2020.01.20-15.54.23 in the example above), while per-scenario
+files reside in a subdirectory named after the scenario (for example,
+"TREE04").  If a given scenario ran more than once (as in "--configs
+'56*TREE04'" above), the directories corresponding to the second and
+subsequent runs of that scenario include a sequence number, for example,
+"TREE04.2", "TREE04.3", and so on.
+
+The most frequently used file in the top-level directory is testid.txt.
+If the test ran in a git repository, then this file contains the commit
+that was tested and any uncommitted changes in diff format.
+
+The most frequently used files in each per-scenario-run directory are:
+
+.config: This file contains the Kconfig options.
+
+Make.out: This contains build output for a specific scenario.
+
+console.log: This contains the console output for a specific scenario.
+	This file may be examined once the kernel has booted, but
+	it might not exist if the build failed.
+
+vmlinux: This contains the kernel, which can be useful with tools like
+	objdump and gdb.
+
+A number of additional files are available, but are less frequently used.
+Many are intended for debugging of rcutorture itself or of its scripting.
+
+As of v5.4, a successful run with the default set of scenarios produces
+the following summary at the end of the run on a 12-CPU system:
+
+SRCU-N ------- 804233 GPs (148.932/s) [srcu: g10008272 f0x0 ]
+SRCU-P ------- 202320 GPs (37.4667/s) [srcud: g1809476 f0x0 ]
+SRCU-t ------- 1122086 GPs (207.794/s) [srcu: g0 f0x0 ]
+SRCU-u ------- 1111285 GPs (205.794/s) [srcud: g1 f0x0 ]
+TASKS01 ------- 19666 GPs (3.64185/s) [tasks: g0 f0x0 ]
+TASKS02 ------- 20541 GPs (3.80389/s) [tasks: g0 f0x0 ]
+TASKS03 ------- 19416 GPs (3.59556/s) [tasks: g0 f0x0 ]
+TINY01 ------- 836134 GPs (154.84/s) [rcu: g0 f0x0 ] n_max_cbs: 34198
+TINY02 ------- 850371 GPs (157.476/s) [rcu: g0 f0x0 ] n_max_cbs: 2631
+TREE01 ------- 162625 GPs (30.1157/s) [rcu: g1124169 f0x0 ]
+TREE02 ------- 333003 GPs (61.6672/s) [rcu: g2647753 f0x0 ] n_max_cbs: 35844
+TREE03 ------- 306623 GPs (56.782/s) [rcu: g2975325 f0x0 ] n_max_cbs: 1496497
+CPU count limited from 16 to 12
+TREE04 ------- 246149 GPs (45.5831/s) [rcu: g1695737 f0x0 ] n_max_cbs: 434961
+TREE05 ------- 314603 GPs (58.2598/s) [rcu: g2257741 f0x2 ] n_max_cbs: 193997
+TREE07 ------- 167347 GPs (30.9902/s) [rcu: g1079021 f0x0 ] n_max_cbs: 478732
+CPU count limited from 16 to 12
+TREE09 ------- 752238 GPs (139.303/s) [rcu: g13075057 f0x0 ] n_max_cbs: 99011

Documentation/admin-guide/kernel-parameters.txt

@@ -4005,6 +4005,15 @@
 			Set threshold of queued RCU callbacks below which
 			batch limiting is re-enabled.
 
+	rcutree.qovld= [KNL]
+			Set threshold of queued RCU callbacks beyond which
+			RCU's force-quiescent-state scan will aggressively
+			enlist help from cond_resched() and sched IPIs to
+			help CPUs more quickly reach quiescent states.
+			Set to less than zero to make this be set based
+			on rcutree.qhimark at boot time and to zero to
+			disable more aggressive help enlistment.
+
 	rcutree.rcu_idle_gp_delay= [KNL]
 			Set wakeup interval for idle CPUs that have
 			RCU callbacks (RCU_FAST_NO_HZ=y).
@@ -4220,6 +4229,12 @@
 	rcupdate.rcu_cpu_stall_suppress= [KNL]
 			Suppress RCU CPU stall warning messages.
 
+	rcupdate.rcu_cpu_stall_suppress_at_boot= [KNL]
+			Suppress RCU CPU stall warning messages and
+			rcutorture writer stall warnings that occur
+			during early boot, that is, during the time
+			before the init task is spawned.
+
 	rcupdate.rcu_cpu_stall_timeout= [KNL]
 			Set timeout for RCU CPU stall warning messages.
 
@@ -4892,6 +4907,10 @@
 			topology updates sent by the hypervisor to this
 			LPAR.
 
+	torture.disable_onoff_at_boot= [KNL]
+			Prevent the CPU-hotplug component of torturing
+			until after init has spawned.
+
 	tp720=		[HW,PS2]
 
 	tpm_suspend_pcr=[HW,TPM]

Documentation/memory-barriers.txt

@@ -185,7 +185,7 @@ As a further example, consider this sequence of events:
 	===============	===============
 	{ A == 1, B == 2, C == 3, P == &A, Q == &C }
 	B = 4;		Q = P;
-	P = &B		D = *Q;
+	P = &B;		D = *Q;
 
 There is an obvious data dependency here, as the value loaded into D depends on
 the address retrieved from P by CPU 2.  At the end of the sequence, any of the
@@ -569,7 +569,7 @@ following sequence of events:
 	{ A == 1, B == 2, C == 3, P == &A, Q == &C }
 	B = 4;
 	<write barrier>
-	WRITE_ONCE(P, &B)
+	WRITE_ONCE(P, &B);
 			      Q = READ_ONCE(P);
 			      D = *Q;
 
@@ -1721,7 +1721,7 @@ of optimizations:
      and WRITE_ONCE() are more selective:  With READ_ONCE() and
      WRITE_ONCE(), the compiler need only forget the contents of the
      indicated memory locations, while with barrier() the compiler must
-     discard the value of all memory locations that it has currented
+     discard the value of all memory locations that it has currently
      cached in any machine registers.  Of course, the compiler must also
      respect the order in which the READ_ONCE()s and WRITE_ONCE()s occur,
      though the CPU of course need not do so.
@@ -1833,7 +1833,7 @@ Aside: In the case of data dependencies, the compiler would be expected
 to issue the loads in the correct order (eg. `a[b]` would have to load
 the value of b before loading a[b]), however there is no guarantee in
 the C specification that the compiler may not speculate the value of b
-(eg. is equal to 1) and load a before b (eg. tmp = a[1]; if (b != 1)
+(eg. is equal to 1) and load a[b] before b (eg. tmp = a[1]; if (b != 1)
 tmp = a[b]; ).  There is also the problem of a compiler reloading b after
 having loaded a[b], thus having a newer copy of b than a[b].  A consensus
 has not yet been reached about these problems, however the READ_ONCE()

fs/nfs/dir.c

@@ -2489,7 +2489,7 @@ static int nfs_access_get_cached_rcu(struct inode *inode, const struct cred *cre
 	rcu_read_lock();
 	if (nfsi->cache_validity & NFS_INO_INVALID_ACCESS)
 		goto out;
-	lh = rcu_dereference(nfsi->access_cache_entry_lru.prev);
+	lh = rcu_dereference(list_tail_rcu(&nfsi->access_cache_entry_lru));
 	cache = list_entry(lh, struct nfs_access_entry, lru);
 	if (lh == &nfsi->access_cache_entry_lru ||
 	    cred_fscmp(cred, cache->cred) != 0)

include/linux/rculist.h

@@ -60,9 +60,9 @@ static inline void INIT_LIST_HEAD_RCU(struct list_head *list)
 #define __list_check_rcu(dummy, cond, extra...)				\
 	({								\
 	check_arg_count_one(extra);					\
-	RCU_LOCKDEP_WARN(!cond && !rcu_read_lock_any_held(),		\
+	RCU_LOCKDEP_WARN(!(cond) && !rcu_read_lock_any_held(),		\
 			 "RCU-list traversed in non-reader section!");	\
-	 })
+	})
 #else
 #define __list_check_rcu(dummy, cond, extra...)				\
 	({ check_arg_count_one(extra); })

include/linux/rcutiny.h

@@ -83,6 +83,7 @@ void rcu_scheduler_starting(void);
 static inline void rcu_scheduler_starting(void) { }
 #endif /* #else #ifndef CONFIG_SRCU */
 static inline void rcu_end_inkernel_boot(void) { }
+static inline bool rcu_inkernel_boot_has_ended(void) { return true; }
 static inline bool rcu_is_watching(void) { return true; }
 static inline void rcu_momentary_dyntick_idle(void) { }
 static inline void kfree_rcu_scheduler_running(void) { }

include/linux/rcutree.h

@@ -54,6 +54,7 @@ void exit_rcu(void);
 void rcu_scheduler_starting(void);
 extern int rcu_scheduler_active __read_mostly;
 void rcu_end_inkernel_boot(void);
+bool rcu_inkernel_boot_has_ended(void);
 bool rcu_is_watching(void);
 #ifndef CONFIG_PREEMPTION
 void rcu_all_qs(void);

include/linux/timer.h

@@ -164,7 +164,7 @@ static inline void destroy_timer_on_stack(struct timer_list *timer) { }
  */
 static inline int timer_pending(const struct timer_list * timer)
 {
-	return timer->entry.pprev != NULL;
+	return !hlist_unhashed_lockless(&timer->entry);
 }
 
 extern void add_timer_on(struct timer_list *timer, int cpu);

include/trace/events/rcu.h

@@ -623,6 +623,34 @@ TRACE_EVENT_RCU(rcu_invoke_kfree_callback,
 		  __entry->rcuname, __entry->rhp, __entry->offset)
 );
 
+/*
+ * Tracepoint for the invocation of a single RCU callback of the special
+ * kfree_bulk() form. The first argument is the RCU flavor, the second
+ * argument is a number of elements in array to free, the third is an
+ * address of the array holding nr_records entries.
+ */
+TRACE_EVENT_RCU(rcu_invoke_kfree_bulk_callback,
+
+	TP_PROTO(const char *rcuname, unsigned long nr_records, void **p),
+
+	TP_ARGS(rcuname, nr_records, p),
+
+	TP_STRUCT__entry(
+		__field(const char *, rcuname)
+		__field(unsigned long, nr_records)
+		__field(void **, p)
+	),
+
+	TP_fast_assign(
+		__entry->rcuname = rcuname;
+		__entry->nr_records = nr_records;
+		__entry->p = p;
+	),
+
+	TP_printk("%s bulk=0x%p nr_records=%lu",
+		__entry->rcuname, __entry->p, __entry->nr_records)
+);
+
 /*
  * Tracepoint for exiting rcu_do_batch after RCU callbacks have been
  * invoked.  The first argument is the name of the RCU flavor,
@@ -712,6 +740,7 @@ TRACE_EVENT_RCU(rcu_torture_read,
  *	"Begin": rcu_barrier() started.
  *	"EarlyExit": rcu_barrier() piggybacked, thus early exit.
  *	"Inc1": rcu_barrier() piggyback check counter incremented.
+ *	"OfflineNoCBQ": rcu_barrier() found offline no-CBs CPU with callbacks.
  *	"OnlineQ": rcu_barrier() found online CPU with callbacks.
  *	"OnlineNQ": rcu_barrier() found online CPU, no callbacks.
  *	"IRQ": An rcu_barrier_callback() callback posted on remote CPU.

kernel/locking/locktorture.c

@@ -618,7 +618,7 @@ static struct lock_torture_ops percpu_rwsem_lock_ops = {
 static int lock_torture_writer(void *arg)
 {
 	struct lock_stress_stats *lwsp = arg;
-	static DEFINE_TORTURE_RANDOM(rand);
+	DEFINE_TORTURE_RANDOM(rand);
 
 	VERBOSE_TOROUT_STRING("lock_torture_writer task started");
 	set_user_nice(current, MAX_NICE);
@@ -655,7 +655,7 @@ static int lock_torture_writer(void *arg)
 static int lock_torture_reader(void *arg)
 {
 	struct lock_stress_stats *lrsp = arg;
-	static DEFINE_TORTURE_RANDOM(rand);
+	DEFINE_TORTURE_RANDOM(rand);
 
 	VERBOSE_TOROUT_STRING("lock_torture_reader task started");
 	set_user_nice(current, MAX_NICE);
@@ -696,15 +696,16 @@ static void __torture_print_stats(char *page,
 		if (statp[i].n_lock_fail)
 			fail = true;
 		sum += statp[i].n_lock_acquired;
-		if (max < statp[i].n_lock_fail)
-			max = statp[i].n_lock_fail;
-		if (min > statp[i].n_lock_fail)
-			min = statp[i].n_lock_fail;
+		if (max < statp[i].n_lock_acquired)
+			max = statp[i].n_lock_acquired;
+		if (min > statp[i].n_lock_acquired)
+			min = statp[i].n_lock_acquired;
 	}
 	page += sprintf(page,
 			"%s:  Total: %lld  Max/Min: %ld/%ld %s  Fail: %d %s\n",
 			write ? "Writes" : "Reads ",
-			sum, max, min, max / 2 > min ? "???" : "",
+			sum, max, min,
+			!onoff_interval && max / 2 > min ? "???" : "",
 			fail, fail ? "!!!" : "");
 	if (fail)
 		atomic_inc(&cxt.n_lock_torture_errors);

kernel/locking/rtmutex.c

@@ -57,7 +57,7 @@ rt_mutex_set_owner(struct rt_mutex *lock, struct task_struct *owner)
 	if (rt_mutex_has_waiters(lock))
 		val |= RT_MUTEX_HAS_WAITERS;
 
-	lock->owner = (struct task_struct *)val;
+	WRITE_ONCE(lock->owner, (struct task_struct *)val);
 }
 
 static inline void clear_rt_mutex_waiters(struct rt_mutex *lock)

kernel/rcu/Makefile

@@ -3,6 +3,10 @@
 # and is generally not a function of system call inputs.
 KCOV_INSTRUMENT := n
 
+ifeq ($(CONFIG_KCSAN),y)
+KBUILD_CFLAGS += -g -fno-omit-frame-pointer
+endif
+
 obj-y += update.o sync.o
 obj-$(CONFIG_TREE_SRCU) += srcutree.o
 obj-$(CONFIG_TINY_SRCU) += srcutiny.o

kernel/rcu/rcu.h

@@ -198,6 +198,13 @@ static inline void debug_rcu_head_unqueue(struct rcu_head *head)
 }
 #endif	/* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
 
+extern int rcu_cpu_stall_suppress_at_boot;
+
+static inline bool rcu_stall_is_suppressed_at_boot(void)
+{
+	return rcu_cpu_stall_suppress_at_boot && !rcu_inkernel_boot_has_ended();
+}
+
 #ifdef CONFIG_RCU_STALL_COMMON
 
 extern int rcu_cpu_stall_ftrace_dump;
@@ -205,6 +212,11 @@ extern int rcu_cpu_stall_suppress;
 extern int rcu_cpu_stall_timeout;
 int rcu_jiffies_till_stall_check(void);
 
+static inline bool rcu_stall_is_suppressed(void)
+{
+	return rcu_stall_is_suppressed_at_boot() || rcu_cpu_stall_suppress;
+}
+
 #define rcu_ftrace_dump_stall_suppress() \
 do { \
 	if (!rcu_cpu_stall_suppress) \
@@ -218,6 +230,11 @@ do { \
 } while (0)
 
 #else /* #endif #ifdef CONFIG_RCU_STALL_COMMON */
+
+static inline bool rcu_stall_is_suppressed(void)
+{
+	return rcu_stall_is_suppressed_at_boot();
+}
 #define rcu_ftrace_dump_stall_suppress()
 #define rcu_ftrace_dump_stall_unsuppress()
 #endif /* #ifdef CONFIG_RCU_STALL_COMMON */
@@ -325,7 +342,8 @@ static inline void rcu_init_levelspread(int *levelspread, const int *levelcnt)
  * Iterate over all possible CPUs in a leaf RCU node.
  */
 #define for_each_leaf_node_possible_cpu(rnp, cpu) \
-	for ((cpu) = cpumask_next((rnp)->grplo - 1, cpu_possible_mask); \
+	for (WARN_ON_ONCE(!rcu_is_leaf_node(rnp)), \
+	     (cpu) = cpumask_next((rnp)->grplo - 1, cpu_possible_mask); \
 	     (cpu) <= rnp->grphi; \
 	     (cpu) = cpumask_next((cpu), cpu_possible_mask))
 
@@ -335,7 +353,8 @@ static inline void rcu_init_levelspread(int *levelspread, const int *levelcnt)
 #define rcu_find_next_bit(rnp, cpu, mask) \
 	((rnp)->grplo + find_next_bit(&(mask), BITS_PER_LONG, (cpu)))
 #define for_each_leaf_node_cpu_mask(rnp, cpu, mask) \
-	for ((cpu) = rcu_find_next_bit((rnp), 0, (mask)); \
+	for (WARN_ON_ONCE(!rcu_is_leaf_node(rnp)), \
+	     (cpu) = rcu_find_next_bit((rnp), 0, (mask)); \
 	     (cpu) <= rnp->grphi; \
 	     (cpu) = rcu_find_next_bit((rnp), (cpu) + 1 - (rnp->grplo), (mask)))
 

kernel/rcu/rcu_segcblist.c

@@ -182,7 +182,7 @@ void rcu_segcblist_offload(struct rcu_segcblist *rsclp)
 bool rcu_segcblist_ready_cbs(struct rcu_segcblist *rsclp)
 {
 	return rcu_segcblist_is_enabled(rsclp) &&
-	       &rsclp->head != rsclp->tails[RCU_DONE_TAIL];
+	       &rsclp->head != READ_ONCE(rsclp->tails[RCU_DONE_TAIL]);
 }
 
 /*
@@ -381,8 +381,6 @@ void rcu_segcblist_insert_pend_cbs(struct rcu_segcblist *rsclp,
 		return; /* Nothing to do. */
 	WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rclp->head);
 	WRITE_ONCE(rsclp->tails[RCU_NEXT_TAIL], rclp->tail);
-	rclp->head = NULL;
-	rclp->tail = &rclp->head;
 }
 
 /*

kernel/rcu/rcuperf.c

@@ -12,6 +12,7 @@
 #include <linux/types.h>
 #include <linux/kernel.h>
 #include <linux/init.h>
+#include <linux/mm.h>
 #include <linux/module.h>
 #include <linux/kthread.h>
 #include <linux/err.h>
@@ -611,6 +612,7 @@ kfree_perf_thread(void *arg)
 	long me = (long)arg;
 	struct kfree_obj *alloc_ptr;
 	u64 start_time, end_time;
+	long long mem_begin, mem_during = 0;
 
 	VERBOSE_PERFOUT_STRING("kfree_perf_thread task started");
 	set_cpus_allowed_ptr(current, cpumask_of(me % nr_cpu_ids));
@@ -626,6 +628,12 @@ kfree_perf_thread(void *arg)
 	}
 
 	do {
+		if (!mem_during) {
+			mem_during = mem_begin = si_mem_available();
+		} else if (loop % (kfree_loops / 4) == 0) {
+			mem_during = (mem_during + si_mem_available()) / 2;
+		}
+
 		for (i = 0; i < kfree_alloc_num; i++) {
 			alloc_ptr = kmalloc(sizeof(struct kfree_obj), GFP_KERNEL);
 			if (!alloc_ptr)
@@ -645,9 +653,11 @@ kfree_perf_thread(void *arg)
 		else
 			b_rcu_gp_test_finished = cur_ops->get_gp_seq();
 
-		pr_alert("Total time taken by all kfree'ers: %llu ns, loops: %d, batches: %ld\n",
+		pr_alert("Total time taken by all kfree'ers: %llu ns, loops: %d, batches: %ld, memory footprint: %lldMB\n",
 		       (unsigned long long)(end_time - start_time), kfree_loops,
-		       rcuperf_seq_diff(b_rcu_gp_test_finished, b_rcu_gp_test_started));
+		       rcuperf_seq_diff(b_rcu_gp_test_finished, b_rcu_gp_test_started),
+		       (mem_begin - mem_during) >> (20 - PAGE_SHIFT));
+
 		if (shutdown) {
 			smp_mb(); /* Assign before wake. */
 			wake_up(&shutdown_wq);

kernel/rcu/rcutorture.c

@@ -339,7 +339,7 @@ rcu_read_delay(struct torture_random_state *rrsp, struct rt_read_seg *rtrsp)
 	 * period, and we want a long delay occasionally to trigger
 	 * force_quiescent_state. */
 
-	if (!rcu_fwd_cb_nodelay &&
+	if (!READ_ONCE(rcu_fwd_cb_nodelay) &&
 	    !(torture_random(rrsp) % (nrealreaders * 2000 * longdelay_ms))) {
 		started = cur_ops->get_gp_seq();
 		ts = rcu_trace_clock_local();
@@ -375,11 +375,12 @@ rcu_torture_pipe_update_one(struct rcu_torture *rp)
 {
 	int i;
 
-	i = rp->rtort_pipe_count;
+	i = READ_ONCE(rp->rtort_pipe_count);
 	if (i > RCU_TORTURE_PIPE_LEN)
 		i = RCU_TORTURE_PIPE_LEN;
 	atomic_inc(&rcu_torture_wcount[i]);
-	if (++rp->rtort_pipe_count >= RCU_TORTURE_PIPE_LEN) {
+	WRITE_ONCE(rp->rtort_pipe_count, i + 1);
+	if (rp->rtort_pipe_count >= RCU_TORTURE_PIPE_LEN) {
 		rp->rtort_mbtest = 0;
 		return true;
 	}
@@ -1015,7 +1016,8 @@ rcu_torture_writer(void *arg)
 			if (i > RCU_TORTURE_PIPE_LEN)
 				i = RCU_TORTURE_PIPE_LEN;
 			atomic_inc(&rcu_torture_wcount[i]);
-			old_rp->rtort_pipe_count++;
+			WRITE_ONCE(old_rp->rtort_pipe_count,
+				   old_rp->rtort_pipe_count + 1);
 			switch (synctype[torture_random(&rand) % nsynctypes]) {
 			case RTWS_DEF_FREE:
 				rcu_torture_writer_state = RTWS_DEF_FREE;
@@ -1067,7 +1069,8 @@ rcu_torture_writer(void *arg)
 		if (stutter_wait("rcu_torture_writer") &&
 		    !READ_ONCE(rcu_fwd_cb_nodelay) &&
 		    !cur_ops->slow_gps &&
-		    !torture_must_stop())
+		    !torture_must_stop() &&
+		    rcu_inkernel_boot_has_ended())
 			for (i = 0; i < ARRAY_SIZE(rcu_tortures); i++)
 				if (list_empty(&rcu_tortures[i].rtort_free) &&
 				    rcu_access_pointer(rcu_torture_current) !=
@@ -1290,7 +1293,7 @@ static bool rcu_torture_one_read(struct torture_random_state *trsp)
 		atomic_inc(&n_rcu_torture_mberror);
 	rtrsp = rcutorture_loop_extend(&readstate, trsp, rtrsp);
 	preempt_disable();
-	pipe_count = p->rtort_pipe_count;
+	pipe_count = READ_ONCE(p->rtort_pipe_count);
 	if (pipe_count > RCU_TORTURE_PIPE_LEN) {
 		/* Should not happen, but... */
 		pipe_count = RCU_TORTURE_PIPE_LEN;
@@ -1404,14 +1407,15 @@ rcu_torture_stats_print(void)
 	int i;
 	long pipesummary[RCU_TORTURE_PIPE_LEN + 1] = { 0 };
 	long batchsummary[RCU_TORTURE_PIPE_LEN + 1] = { 0 };
+	struct rcu_torture *rtcp;
 	static unsigned long rtcv_snap = ULONG_MAX;
 	static bool splatted;
 	struct task_struct *wtp;
 
 	for_each_possible_cpu(cpu) {
 		for (i = 0; i < RCU_TORTURE_PIPE_LEN + 1; i++) {
-			pipesummary[i] += per_cpu(rcu_torture_count, cpu)[i];
-			batchsummary[i] += per_cpu(rcu_torture_batch, cpu)[i];
+			pipesummary[i] += READ_ONCE(per_cpu(rcu_torture_count, cpu)[i]);
+			batchsummary[i] += READ_ONCE(per_cpu(rcu_torture_batch, cpu)[i]);
 		}
 	}
 	for (i = RCU_TORTURE_PIPE_LEN - 1; i >= 0; i--) {
@@ -1420,9 +1424,10 @@ rcu_torture_stats_print(void)
 	}
 
 	pr_alert("%s%s ", torture_type, TORTURE_FLAG);
+	rtcp = rcu_access_pointer(rcu_torture_current);
 	pr_cont("rtc: %p %s: %lu tfle: %d rta: %d rtaf: %d rtf: %d ",
-		rcu_torture_current,
-		rcu_torture_current ? "ver" : "VER",
+		rtcp,
+		rtcp && !rcu_stall_is_suppressed_at_boot() ? "ver" : "VER",
 		rcu_torture_current_version,
 		list_empty(&rcu_torture_freelist),
 		atomic_read(&n_rcu_torture_alloc),
@@ -1478,7 +1483,8 @@ rcu_torture_stats_print(void)
 	if (cur_ops->stats)
 		cur_ops->stats();
 	if (rtcv_snap == rcu_torture_current_version &&
-	    rcu_torture_current != NULL) {
+	    rcu_access_pointer(rcu_torture_current) &&
+	    !rcu_stall_is_suppressed()) {
 		int __maybe_unused flags = 0;
 		unsigned long __maybe_unused gp_seq = 0;
 
@@ -1993,8 +1999,11 @@ static int rcu_torture_fwd_prog(void *args)
 		schedule_timeout_interruptible(fwd_progress_holdoff * HZ);
 		WRITE_ONCE(rcu_fwd_emergency_stop, false);
 		register_oom_notifier(&rcutorture_oom_nb);
-		rcu_torture_fwd_prog_nr(rfp, &tested, &tested_tries);
-		rcu_torture_fwd_prog_cr(rfp);
+		if (!IS_ENABLED(CONFIG_TINY_RCU) ||
+		    rcu_inkernel_boot_has_ended())
+			rcu_torture_fwd_prog_nr(rfp, &tested, &tested_tries);
+		if (rcu_inkernel_boot_has_ended())
+			rcu_torture_fwd_prog_cr(rfp);
 		unregister_oom_notifier(&rcutorture_oom_nb);
 
 		/* Avoid slow periods, better to test when busy. */
@@ -2044,6 +2053,14 @@ static void rcu_torture_barrier_cbf(struct rcu_head *rcu)
 	atomic_inc(&barrier_cbs_invoked);
 }
 
+/* IPI handler to get callback posted on desired CPU, if online. */
+static void rcu_torture_barrier1cb(void *rcu_void)
+{
+	struct rcu_head *rhp = rcu_void;
+
+	cur_ops->call(rhp, rcu_torture_barrier_cbf);
+}
+
 /* kthread function to register callbacks used to test RCU barriers. */
 static int rcu_torture_barrier_cbs(void *arg)
 {
@@ -2067,9 +2084,11 @@ static int rcu_torture_barrier_cbs(void *arg)
 		 * The above smp_load_acquire() ensures barrier_phase load
 		 * is ordered before the following ->call().
 		 */
-		local_irq_disable(); /* Just to test no-irq call_rcu(). */
-		cur_ops->call(&rcu, rcu_torture_barrier_cbf);
-		local_irq_enable();
+		if (smp_call_function_single(myid, rcu_torture_barrier1cb,
+					     &rcu, 1)) {
+			// IPI failed, so use direct call from current CPU.
+			cur_ops->call(&rcu, rcu_torture_barrier_cbf);
+		}
 		if (atomic_dec_and_test(&barrier_cbs_count))
 			wake_up(&barrier_wq);
 	} while (!torture_must_stop());
@@ -2105,7 +2124,21 @@ static int rcu_torture_barrier(void *arg)
 			pr_err("barrier_cbs_invoked = %d, n_barrier_cbs = %d\n",
 			       atomic_read(&barrier_cbs_invoked),
 			       n_barrier_cbs);
-			WARN_ON_ONCE(1);
+			WARN_ON(1);
+			// Wait manually for the remaining callbacks
+			i = 0;
+			do {
+				if (WARN_ON(i++ > HZ))
+					i = INT_MIN;
+				schedule_timeout_interruptible(1);
+				cur_ops->cb_barrier();
+			} while (atomic_read(&barrier_cbs_invoked) !=
+				 n_barrier_cbs &&
+				 !torture_must_stop());
+			smp_mb(); // Can't trust ordering if broken.
+			if (!torture_must_stop())
+				pr_err("Recovered: barrier_cbs_invoked = %d\n",
+				       atomic_read(&barrier_cbs_invoked));
 		} else {
 			n_barrier_successes++;
 		}

kernel/rcu/srcutree.c

@@ -5,7 +5,7 @@
  * Copyright (C) IBM Corporation, 2006
  * Copyright (C) Fujitsu, 2012
  *
- * Author: Paul McKenney <paulmck@linux.ibm.com>
+ * Authors: Paul McKenney <paulmck@linux.ibm.com>
  *	   Lai Jiangshan <laijs@cn.fujitsu.com>
  *
  * For detailed explanation of Read-Copy Update mechanism see -
@@ -450,7 +450,7 @@ static void srcu_gp_start(struct srcu_struct *ssp)
 	spin_unlock_rcu_node(sdp);  /* Interrupts remain disabled. */
 	smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */
 	rcu_seq_start(&ssp->srcu_gp_seq);
-	state = rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq));
+	state = rcu_seq_state(ssp->srcu_gp_seq);
 	WARN_ON_ONCE(state != SRCU_STATE_SCAN1);
 }
 
@@ -534,7 +534,7 @@ static void srcu_gp_end(struct srcu_struct *ssp)
 	rcu_seq_end(&ssp->srcu_gp_seq);
 	gpseq = rcu_seq_current(&ssp->srcu_gp_seq);
 	if (ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, gpseq))
-		ssp->srcu_gp_seq_needed_exp = gpseq;
+		WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, gpseq);
 	spin_unlock_irq_rcu_node(ssp);
 	mutex_unlock(&ssp->srcu_gp_mutex);
 	/* A new grace period can start at this point.  But only one. */
@@ -550,7 +550,7 @@ static void srcu_gp_end(struct srcu_struct *ssp)
 		snp->srcu_have_cbs[idx] = gpseq;
 		rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
 		if (ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, gpseq))
-			snp->srcu_gp_seq_needed_exp = gpseq;
+			WRITE_ONCE(snp->srcu_gp_seq_needed_exp, gpseq);
 		mask = snp->srcu_data_have_cbs[idx];
 		snp->srcu_data_have_cbs[idx] = 0;
 		spin_unlock_irq_rcu_node(snp);
@@ -614,7 +614,7 @@ static void srcu_funnel_exp_start(struct srcu_struct *ssp, struct srcu_node *snp
 	}
 	spin_lock_irqsave_rcu_node(ssp, flags);
 	if (ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, s))
-		ssp->srcu_gp_seq_needed_exp = s;
+		WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, s);
 	spin_unlock_irqrestore_rcu_node(ssp, flags);
 }
 
@@ -660,7 +660,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
 		if (snp == sdp->mynode)
 			snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
 		if (!do_norm && ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, s))
-			snp->srcu_gp_seq_needed_exp = s;
+			WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
 		spin_unlock_irqrestore_rcu_node(snp, flags);
 	}
 
@@ -674,7 +674,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
 		smp_store_release(&ssp->srcu_gp_seq_needed, s); /*^^^*/
 	}
 	if (!do_norm && ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, s))
-		ssp->srcu_gp_seq_needed_exp = s;
+		WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, s);
 
 	/* If grace period not already done and none in progress, start it. */
 	if (!rcu_seq_done(&ssp->srcu_gp_seq, s) &&
@@ -1079,7 +1079,7 @@ EXPORT_SYMBOL_GPL(srcu_barrier);
  */
 unsigned long srcu_batches_completed(struct srcu_struct *ssp)
 {
-	return ssp->srcu_idx;
+	return READ_ONCE(ssp->srcu_idx);
 }
 EXPORT_SYMBOL_GPL(srcu_batches_completed);
 
@@ -1130,7 +1130,9 @@ static void srcu_advance_state(struct srcu_struct *ssp)
 			return; /* readers present, retry later. */
 		}
 		srcu_flip(ssp);
+		spin_lock_irq_rcu_node(ssp);
 		rcu_seq_set_state(&ssp->srcu_gp_seq, SRCU_STATE_SCAN2);
+		spin_unlock_irq_rcu_node(ssp);
 	}
 
 	if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) == SRCU_STATE_SCAN2) {

kernel/rcu/tree.c

 // SPDX-License-Identifier: GPL-2.0+
 /*
- * Read-Copy Update mechanism for mutual exclusion
+ * Read-Copy Update mechanism for mutual exclusion (tree-based version)
  *
  * Copyright IBM Corporation, 2008
  *
  * Authors: Dipankar Sarma <dipankar@in.ibm.com>
  *	    Manfred Spraul <manfred@colorfullife.com>
- *	    Paul E. McKenney <paulmck@linux.ibm.com> Hierarchical version
+ *	    Paul E. McKenney <paulmck@linux.ibm.com>
  *
  * Based on the original work by Paul McKenney <paulmck@linux.ibm.com>
  * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
@@ -150,6 +150,7 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
 static void invoke_rcu_core(void);
 static void rcu_report_exp_rdp(struct rcu_data *rdp);
 static void sync_sched_exp_online_cleanup(int cpu);
+static void check_cb_ovld_locked(struct rcu_data *rdp, struct rcu_node *rnp);
 
 /* rcuc/rcub kthread realtime priority */
 static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 1 : 0;
@@ -342,14 +343,17 @@ bool rcu_eqs_special_set(int cpu)
 {
 	int old;
 	int new;
+	int new_old;
 	struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
 
+	new_old = atomic_read(&rdp->dynticks);
 	do {
-		old = atomic_read(&rdp->dynticks);
+		old = new_old;
 		if (old & RCU_DYNTICK_CTRL_CTR)
 			return false;
 		new = old | RCU_DYNTICK_CTRL_MASK;
-	} while (atomic_cmpxchg(&rdp->dynticks, old, new) != old);
+		new_old = atomic_cmpxchg(&rdp->dynticks, old, new);
+	} while (new_old != old);
 	return true;
 }
 
@@ -410,10 +414,15 @@ static long blimit = DEFAULT_RCU_BLIMIT;
 static long qhimark = DEFAULT_RCU_QHIMARK;
 #define DEFAULT_RCU_QLOMARK 100   /* Once only this many pending, use blimit. */
 static long qlowmark = DEFAULT_RCU_QLOMARK;
+#define DEFAULT_RCU_QOVLD_MULT 2
+#define DEFAULT_RCU_QOVLD (DEFAULT_RCU_QOVLD_MULT * DEFAULT_RCU_QHIMARK)
+static long qovld = DEFAULT_RCU_QOVLD; /* If this many pending, hammer QS. */
+static long qovld_calc = -1;	  /* No pre-initialization lock acquisitions! */
 
 module_param(blimit, long, 0444);
 module_param(qhimark, long, 0444);
 module_param(qlowmark, long, 0444);
+module_param(qovld, long, 0444);
 
 static ulong jiffies_till_first_fqs = ULONG_MAX;
 static ulong jiffies_till_next_fqs = ULONG_MAX;
@@ -818,11 +827,12 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
 		incby = 1;
 	} else if (tick_nohz_full_cpu(rdp->cpu) &&
 		   rdp->dynticks_nmi_nesting == DYNTICK_IRQ_NONIDLE &&
-		   READ_ONCE(rdp->rcu_urgent_qs) && !rdp->rcu_forced_tick) {
+		   READ_ONCE(rdp->rcu_urgent_qs) &&
+		   !READ_ONCE(rdp->rcu_forced_tick)) {
 		raw_spin_lock_rcu_node(rdp->mynode);
 		// Recheck under lock.
 		if (rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
-			rdp->rcu_forced_tick = true;
+			WRITE_ONCE(rdp->rcu_forced_tick, true);
 			tick_dep_set_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
 		}
 		raw_spin_unlock_rcu_node(rdp->mynode);
@@ -899,7 +909,7 @@ static void rcu_disable_urgency_upon_qs(struct rcu_data *rdp)
 	WRITE_ONCE(rdp->rcu_need_heavy_qs, false);
 	if (tick_nohz_full_cpu(rdp->cpu) && rdp->rcu_forced_tick) {
 		tick_dep_clear_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
-		rdp->rcu_forced_tick = false;
+		WRITE_ONCE(rdp->rcu_forced_tick, false);
 	}
 }
 
@@ -1072,7 +1082,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 	rnhqp = &per_cpu(rcu_data.rcu_need_heavy_qs, rdp->cpu);
 	if (!READ_ONCE(*rnhqp) &&
 	    (time_after(jiffies, rcu_state.gp_start + jtsq * 2) ||
-	     time_after(jiffies, rcu_state.jiffies_resched))) {
+	     time_after(jiffies, rcu_state.jiffies_resched) ||
+	     rcu_state.cbovld)) {
 		WRITE_ONCE(*rnhqp, true);
 		/* Store rcu_need_heavy_qs before rcu_urgent_qs. */
 		smp_store_release(ruqp, true);
@@ -1089,8 +1100,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 	 * So hit them over the head with the resched_cpu() hammer!
 	 */
 	if (tick_nohz_full_cpu(rdp->cpu) &&
-		   time_after(jiffies,
-			      READ_ONCE(rdp->last_fqs_resched) + jtsq * 3)) {
+	    (time_after(jiffies, READ_ONCE(rdp->last_fqs_resched) + jtsq * 3) ||
+	     rcu_state.cbovld)) {
 		WRITE_ONCE(*ruqp, true);
 		resched_cpu(rdp->cpu);
 		WRITE_ONCE(rdp->last_fqs_resched, jiffies);
@@ -1126,8 +1137,9 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 			      unsigned long gp_seq_req, const char *s)
 {
-	trace_rcu_future_grace_period(rcu_state.name, rnp->gp_seq, gp_seq_req,
-				      rnp->level, rnp->grplo, rnp->grphi, s);
+	trace_rcu_future_grace_period(rcu_state.name, READ_ONCE(rnp->gp_seq),
+				      gp_seq_req, rnp->level,
+				      rnp->grplo, rnp->grphi, s);
 }
 
 /*
@@ -1174,7 +1186,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp,
 					  TPS("Prestarted"));
 			goto unlock_out;
 		}
-		rnp->gp_seq_needed = gp_seq_req;
+		WRITE_ONCE(rnp->gp_seq_needed, gp_seq_req);
 		if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) {
 			/*
 			 * We just marked the leaf or internal node, and a
@@ -1199,18 +1211,18 @@ static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp,
 	}
 	trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startedroot"));
 	WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags | RCU_GP_FLAG_INIT);
-	rcu_state.gp_req_activity = jiffies;
-	if (!rcu_state.gp_kthread) {
+	WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
+	if (!READ_ONCE(rcu_state.gp_kthread)) {
 		trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("NoGPkthread"));
 		goto unlock_out;
 	}
-	trace_rcu_grace_period(rcu_state.name, READ_ONCE(rcu_state.gp_seq), TPS("newreq"));
+	trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("newreq"));
 	ret = true;  /* Caller must wake GP kthread. */
 unlock_out:
 	/* Push furthest requested GP to leaf node and rcu_data structure. */
 	if (ULONG_CMP_LT(gp_seq_req, rnp->gp_seq_needed)) {
-		rnp_start->gp_seq_needed = rnp->gp_seq_needed;
-		rdp->gp_seq_needed = rnp->gp_seq_needed;
+		WRITE_ONCE(rnp_start->gp_seq_needed, rnp->gp_seq_needed);
+		WRITE_ONCE(rdp->gp_seq_needed, rnp->gp_seq_needed);
 	}
 	if (rnp != rnp_start)
 		raw_spin_unlock_rcu_node(rnp);
@@ -1235,12 +1247,13 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp)
 }
 
 /*
- * Awaken the grace-period kthread.  Don't do a self-awaken (unless in
- * an interrupt or softirq handler), and don't bother awakening when there
- * is nothing for the grace-period kthread to do (as in several CPUs raced
- * to awaken, and we lost), and finally don't try to awaken a kthread that
- * has not yet been created.  If all those checks are passed, track some
- * debug information and awaken.
+ * Awaken the grace-period kthread.  Don't do a self-awaken (unless in an
+ * interrupt or softirq handler, in which case we just might immediately
+ * sleep upon return, resulting in a grace-period hang), and don't bother
+ * awakening when there is nothing for the grace-period kthread to do
+ * (as in several CPUs raced to awaken, we lost), and finally don't try
+ * to awaken a kthread that has not yet been created.  If all those checks
+ * are passed, track some debug information and awaken.
  *
  * So why do the self-wakeup when in an interrupt or softirq handler
  * in the grace-period kthread's context?  Because the kthread might have
@@ -1250,10 +1263,10 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp)
  */
 static void rcu_gp_kthread_wake(void)
 {
-	if ((current == rcu_state.gp_kthread &&
-	     !in_irq() && !in_serving_softirq()) ||
-	    !READ_ONCE(rcu_state.gp_flags) ||
-	    !rcu_state.gp_kthread)
+	struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
+
+	if ((current == t && !in_irq() && !in_serving_softirq()) ||
+	    !READ_ONCE(rcu_state.gp_flags) || !t)
 		return;
 	WRITE_ONCE(rcu_state.gp_wake_time, jiffies);
 	WRITE_ONCE(rcu_state.gp_wake_seq, READ_ONCE(rcu_state.gp_seq));
@@ -1321,7 +1334,7 @@ static void rcu_accelerate_cbs_unlocked(struct rcu_node *rnp,
 
 	rcu_lockdep_assert_cblist_protected(rdp);
 	c = rcu_seq_snap(&rcu_state.gp_seq);
-	if (!rdp->gpwrap && ULONG_CMP_GE(rdp->gp_seq_needed, c)) {
+	if (!READ_ONCE(rdp->gpwrap) && ULONG_CMP_GE(rdp->gp_seq_needed, c)) {
 		/* Old request still live, so mark recent callbacks. */
 		(void)rcu_segcblist_accelerate(&rdp->cblist, c);
 		return;
@@ -1386,7 +1399,7 @@ static void __maybe_unused rcu_advance_cbs_nowake(struct rcu_node *rnp,
 static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
 {
 	bool ret = false;
-	bool need_gp;
+	bool need_qs;
 	const bool offloaded = IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
 			       rcu_segcblist_is_offloaded(&rdp->cblist);
 
@@ -1400,10 +1413,13 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
 	    unlikely(READ_ONCE(rdp->gpwrap))) {
 		if (!offloaded)
 			ret = rcu_advance_cbs(rnp, rdp); /* Advance CBs. */
+		rdp->core_needs_qs = false;
 		trace_rcu_grace_period(rcu_state.name, rdp->gp_seq, TPS("cpuend"));
 	} else {
 		if (!offloaded)
 			ret = rcu_accelerate_cbs(rnp, rdp); /* Recent CBs. */
+		if (rdp->core_needs_qs)
+			rdp->core_needs_qs = !!(rnp->qsmask & rdp->grpmask);
 	}
 
 	/* Now handle the beginnings of any new-to-this-CPU grace periods. */
@@ -1415,14 +1431,14 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
 		 * go looking for one.
 		 */
 		trace_rcu_grace_period(rcu_state.name, rnp->gp_seq, TPS("cpustart"));
-		need_gp = !!(rnp->qsmask & rdp->grpmask);
-		rdp->cpu_no_qs.b.norm = need_gp;
-		rdp->core_needs_qs = need_gp;
+		need_qs = !!(rnp->qsmask & rdp->grpmask);
+		rdp->cpu_no_qs.b.norm = need_qs;
+		rdp->core_needs_qs = need_qs;
 		zero_cpu_stall_ticks(rdp);
 	}
 	rdp->gp_seq = rnp->gp_seq;  /* Remember new grace-period state. */
 	if (ULONG_CMP_LT(rdp->gp_seq_needed, rnp->gp_seq_needed) || rdp->gpwrap)
-		rdp->gp_seq_needed = rnp->gp_seq_needed;
+		WRITE_ONCE(rdp->gp_seq_needed, rnp->gp_seq_needed);
 	WRITE_ONCE(rdp->gpwrap, false);
 	rcu_gpnum_ovf(rnp, rdp);
 	return ret;
@@ -1651,8 +1667,7 @@ static void rcu_gp_fqs_loop(void)
 			WRITE_ONCE(rcu_state.jiffies_kick_kthreads,
 				   jiffies + (j ? 3 * j : 2));
 		}
-		trace_rcu_grace_period(rcu_state.name,
-				       READ_ONCE(rcu_state.gp_seq),
+		trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
 				       TPS("fqswait"));
 		rcu_state.gp_state = RCU_GP_WAIT_FQS;
 		ret = swait_event_idle_timeout_exclusive(
@@ -1666,13 +1681,11 @@ static void rcu_gp_fqs_loop(void)
 		/* If time for quiescent-state forcing, do it. */
 		if (ULONG_CMP_GE(jiffies, rcu_state.jiffies_force_qs) ||
 		    (gf & RCU_GP_FLAG_FQS)) {
-			trace_rcu_grace_period(rcu_state.name,
-					       READ_ONCE(rcu_state.gp_seq),
+			trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
 					       TPS("fqsstart"));
 			rcu_gp_fqs(first_gp_fqs);
 			first_gp_fqs = false;
-			trace_rcu_grace_period(rcu_state.name,
-					       READ_ONCE(rcu_state.gp_seq),
+			trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
 					       TPS("fqsend"));
 			cond_resched_tasks_rcu_qs();
 			WRITE_ONCE(rcu_state.gp_activity, jiffies);
@@ -1683,8 +1696,7 @@ static void rcu_gp_fqs_loop(void)
 			cond_resched_tasks_rcu_qs();
 			WRITE_ONCE(rcu_state.gp_activity, jiffies);
 			WARN_ON(signal_pending(current));
-			trace_rcu_grace_period(rcu_state.name,
-					       READ_ONCE(rcu_state.gp_seq),
+			trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
 					       TPS("fqswaitsig"));
 			ret = 1; /* Keep old FQS timing. */
 			j = jiffies;
@@ -1701,8 +1713,9 @@ static void rcu_gp_fqs_loop(void)
  */
 static void rcu_gp_cleanup(void)
 {
-	unsigned long gp_duration;
+	int cpu;
 	bool needgp = false;
+	unsigned long gp_duration;
 	unsigned long new_gp_seq;
 	bool offloaded;
 	struct rcu_data *rdp;
@@ -1748,6 +1761,12 @@ static void rcu_gp_cleanup(void)
 			needgp = __note_gp_changes(rnp, rdp) || needgp;
 		/* smp_mb() provided by prior unlock-lock pair. */
 		needgp = rcu_future_gp_cleanup(rnp) || needgp;
+		// Reset overload indication for CPUs no longer overloaded
+		if (rcu_is_leaf_node(rnp))
+			for_each_leaf_node_cpu_mask(rnp, cpu, rnp->cbovldmask) {
+				rdp = per_cpu_ptr(&rcu_data, cpu);
+				check_cb_ovld_locked(rdp, rnp);
+			}
 		sq = rcu_nocb_gp_get(rnp);
 		raw_spin_unlock_irq_rcu_node(rnp);
 		rcu_nocb_gp_cleanup(sq);
@@ -1774,9 +1793,9 @@ static void rcu_gp_cleanup(void)
 		    rcu_segcblist_is_offloaded(&rdp->cblist);
 	if ((offloaded || !rcu_accelerate_cbs(rnp, rdp)) && needgp) {
 		WRITE_ONCE(rcu_state.gp_flags, RCU_GP_FLAG_INIT);
-		rcu_state.gp_req_activity = jiffies;
+		WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
 		trace_rcu_grace_period(rcu_state.name,
-				       READ_ONCE(rcu_state.gp_seq),
+				       rcu_state.gp_seq,
 				       TPS("newreq"));
 	} else {
 		WRITE_ONCE(rcu_state.gp_flags,
@@ -1795,8 +1814,7 @@ static int __noreturn rcu_gp_kthread(void *unused)
 
 		/* Handle grace-period start. */
 		for (;;) {
-			trace_rcu_grace_period(rcu_state.name,
-					       READ_ONCE(rcu_state.gp_seq),
+			trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
 					       TPS("reqwait"));
 			rcu_state.gp_state = RCU_GP_WAIT_GPS;
 			swait_event_idle_exclusive(rcu_state.gp_wq,
@@ -1809,8 +1827,7 @@ static int __noreturn rcu_gp_kthread(void *unused)
 			cond_resched_tasks_rcu_qs();
 			WRITE_ONCE(rcu_state.gp_activity, jiffies);
 			WARN_ON(signal_pending(current));
-			trace_rcu_grace_period(rcu_state.name,
-					       READ_ONCE(rcu_state.gp_seq),
+			trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
 					       TPS("reqwaitsig"));
 		}
 
@@ -1881,7 +1898,7 @@ static void rcu_report_qs_rnp(unsigned long mask, struct rcu_node *rnp,
 		WARN_ON_ONCE(oldmask); /* Any child must be all zeroed! */
 		WARN_ON_ONCE(!rcu_is_leaf_node(rnp) &&
 			     rcu_preempt_blocked_readers_cgp(rnp));
-		rnp->qsmask &= ~mask;
+		WRITE_ONCE(rnp->qsmask, rnp->qsmask & ~mask);
 		trace_rcu_quiescent_state_report(rcu_state.name, rnp->gp_seq,
 						 mask, rnp->qsmask, rnp->level,
 						 rnp->grplo, rnp->grphi,
@@ -1904,7 +1921,7 @@ static void rcu_report_qs_rnp(unsigned long mask, struct rcu_node *rnp,
 		rnp_c = rnp;
 		rnp = rnp->parent;
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
-		oldmask = rnp_c->qsmask;
+		oldmask = READ_ONCE(rnp_c->qsmask);
 	}
 
 	/*
@@ -1987,6 +2004,8 @@ rcu_report_qs_rdp(int cpu, struct rcu_data *rdp)
 		return;
 	}
 	mask = rdp->grpmask;
+	if (rdp->cpu == smp_processor_id())
+		rdp->core_needs_qs = false;
 	if ((rnp->qsmask & mask) == 0) {
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	} else {
@@ -2052,7 +2071,7 @@ int rcutree_dying_cpu(unsigned int cpu)
 		return 0;
 
 	blkd = !!(rnp->qsmask & rdp->grpmask);
-	trace_rcu_grace_period(rcu_state.name, rnp->gp_seq,
+	trace_rcu_grace_period(rcu_state.name, READ_ONCE(rnp->gp_seq),
 			       blkd ? TPS("cpuofl") : TPS("cpuofl-bgp"));
 	return 0;
 }
@@ -2294,10 +2313,13 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
 	struct rcu_data *rdp;
 	struct rcu_node *rnp;
 
+	rcu_state.cbovld = rcu_state.cbovldnext;
+	rcu_state.cbovldnext = false;
 	rcu_for_each_leaf_node(rnp) {
 		cond_resched_tasks_rcu_qs();
 		mask = 0;
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
+		rcu_state.cbovldnext |= !!rnp->cbovldmask;
 		if (rnp->qsmask == 0) {
 			if (!IS_ENABLED(CONFIG_PREEMPT_RCU) ||
 			    rcu_preempt_blocked_readers_cgp(rnp)) {
@@ -2579,11 +2601,48 @@ static void rcu_leak_callback(struct rcu_head *rhp)
 }
 
 /*
- * Helper function for call_rcu() and friends.  The cpu argument will
- * normally be -1, indicating "currently running CPU".  It may specify
- * a CPU only if that CPU is a no-CBs CPU.  Currently, only rcu_barrier()
- * is expected to specify a CPU.
+ * Check and if necessary update the leaf rcu_node structure's
+ * ->cbovldmask bit corresponding to the current CPU based on that CPU's
+ * number of queued RCU callbacks.  The caller must hold the leaf rcu_node
+ * structure's ->lock.
  */
+static void check_cb_ovld_locked(struct rcu_data *rdp, struct rcu_node *rnp)
+{
+	raw_lockdep_assert_held_rcu_node(rnp);
+	if (qovld_calc <= 0)
+		return; // Early boot and wildcard value set.
+	if (rcu_segcblist_n_cbs(&rdp->cblist) >= qovld_calc)
+		WRITE_ONCE(rnp->cbovldmask, rnp->cbovldmask | rdp->grpmask);
+	else
+		WRITE_ONCE(rnp->cbovldmask, rnp->cbovldmask & ~rdp->grpmask);
+}
+
+/*
+ * Check and if necessary update the leaf rcu_node structure's
+ * ->cbovldmask bit corresponding to the current CPU based on that CPU's
+ * number of queued RCU callbacks.  No locks need be held, but the
+ * caller must have disabled interrupts.
+ *
+ * Note that this function ignores the possibility that there are a lot
+ * of callbacks all of which have already seen the end of their respective
+ * grace periods.  This omission is due to the need for no-CBs CPUs to
+ * be holding ->nocb_lock to do this check, which is too heavy for a
+ * common-case operation.
+ */
+static void check_cb_ovld(struct rcu_data *rdp)
+{
+	struct rcu_node *const rnp = rdp->mynode;
+
+	if (qovld_calc <= 0 ||
+	    ((rcu_segcblist_n_cbs(&rdp->cblist) >= qovld_calc) ==
+	     !!(READ_ONCE(rnp->cbovldmask) & rdp->grpmask)))
+		return; // Early boot wildcard value or already set correctly.
+	raw_spin_lock_rcu_node(rnp);
+	check_cb_ovld_locked(rdp, rnp);
+	raw_spin_unlock_rcu_node(rnp);
+}
+
+/* Helper function for call_rcu() and friends.  */
 static void
 __call_rcu(struct rcu_head *head, rcu_callback_t func)
 {
@@ -2621,9 +2680,10 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
 			rcu_segcblist_init(&rdp->cblist);
 	}
 
+	check_cb_ovld(rdp);
 	if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags))
 		return; // Enqueued onto ->nocb_bypass, so just leave.
-	/* If we get here, rcu_nocb_try_bypass() acquired ->nocb_lock. */
+	// If no-CBs CPU gets here, rcu_nocb_try_bypass() acquired ->nocb_lock.
 	rcu_segcblist_enqueue(&rdp->cblist, head);
 	if (__is_kfree_rcu_offset((unsigned long)func))
 		trace_rcu_kfree_callback(rcu_state.name, head,
@@ -2689,22 +2749,47 @@ EXPORT_SYMBOL_GPL(call_rcu);
 #define KFREE_DRAIN_JIFFIES (HZ / 50)
 #define KFREE_N_BATCHES 2
 
+/*
+ * This macro defines how many entries the "records" array
+ * will contain. It is based on the fact that the size of
+ * kfree_rcu_bulk_data structure becomes exactly one page.
+ */
+#define KFREE_BULK_MAX_ENTR ((PAGE_SIZE / sizeof(void *)) - 3)
+
+/**
+ * struct kfree_rcu_bulk_data - single block to store kfree_rcu() pointers
+ * @nr_records: Number of active pointers in the array
+ * @records: Array of the kfree_rcu() pointers
+ * @next: Next bulk object in the block chain
+ * @head_free_debug: For debug, when CONFIG_DEBUG_OBJECTS_RCU_HEAD is set
+ */
+struct kfree_rcu_bulk_data {
+	unsigned long nr_records;
+	void *records[KFREE_BULK_MAX_ENTR];
+	struct kfree_rcu_bulk_data *next;
+	struct rcu_head *head_free_debug;
+};
+
 /**
  * struct kfree_rcu_cpu_work - single batch of kfree_rcu() requests
  * @rcu_work: Let queue_rcu_work() invoke workqueue handler after grace period
  * @head_free: List of kfree_rcu() objects waiting for a grace period
+ * @bhead_free: Bulk-List of kfree_rcu() objects waiting for a grace period
  * @krcp: Pointer to @kfree_rcu_cpu structure
  */
 
 struct kfree_rcu_cpu_work {
 	struct rcu_work rcu_work;
 	struct rcu_head *head_free;
+	struct kfree_rcu_bulk_data *bhead_free;
 	struct kfree_rcu_cpu *krcp;
 };
 
 /**
  * struct kfree_rcu_cpu - batch up kfree_rcu() requests for RCU grace period
  * @head: List of kfree_rcu() objects not yet waiting for a grace period
+ * @bhead: Bulk-List of kfree_rcu() objects not yet waiting for a grace period
+ * @bcached: Keeps at most one object for later reuse when build chain blocks
  * @krw_arr: Array of batches of kfree_rcu() objects waiting for a grace period
  * @lock: Synchronize access to this structure
  * @monitor_work: Promote @head to @head_free after KFREE_DRAIN_JIFFIES
@@ -2718,6 +2803,8 @@ struct kfree_rcu_cpu_work {
  */
 struct kfree_rcu_cpu {
 	struct rcu_head *head;
+	struct kfree_rcu_bulk_data *bhead;
+	struct kfree_rcu_bulk_data *bcached;
 	struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
 	spinlock_t lock;
 	struct delayed_work monitor_work;
@@ -2727,14 +2814,24 @@ struct kfree_rcu_cpu {
 
 static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc);
 
+static __always_inline void
+debug_rcu_head_unqueue_bulk(struct rcu_head *head)
+{
+#ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
+	for (; head; head = head->next)
+		debug_rcu_head_unqueue(head);
+#endif
+}
+
 /*
  * This function is invoked in workqueue context after a grace period.
- * It frees all the objects queued on ->head_free.
+ * It frees all the objects queued on ->bhead_free or ->head_free.
  */
 static void kfree_rcu_work(struct work_struct *work)
 {
 	unsigned long flags;
 	struct rcu_head *head, *next;
+	struct kfree_rcu_bulk_data *bhead, *bnext;
 	struct kfree_rcu_cpu *krcp;
 	struct kfree_rcu_cpu_work *krwp;
 
@@ -2744,22 +2841,44 @@ static void kfree_rcu_work(struct work_struct *work)
 	spin_lock_irqsave(&krcp->lock, flags);
 	head = krwp->head_free;
 	krwp->head_free = NULL;
+	bhead = krwp->bhead_free;
+	krwp->bhead_free = NULL;
 	spin_unlock_irqrestore(&krcp->lock, flags);
 
-	// List "head" is now private, so traverse locklessly.
+	/* "bhead" is now private, so traverse locklessly. */
+	for (; bhead; bhead = bnext) {
+		bnext = bhead->next;
+
+		debug_rcu_head_unqueue_bulk(bhead->head_free_debug);
+
+		rcu_lock_acquire(&rcu_callback_map);
+		trace_rcu_invoke_kfree_bulk_callback(rcu_state.name,
+			bhead->nr_records, bhead->records);
+
+		kfree_bulk(bhead->nr_records, bhead->records);
+		rcu_lock_release(&rcu_callback_map);
+
+		if (cmpxchg(&krcp->bcached, NULL, bhead))
+			free_page((unsigned long) bhead);
+
+		cond_resched_tasks_rcu_qs();
+	}
+
+	/*
+	 * Emergency case only. It can happen under low memory
+	 * condition when an allocation gets failed, so the "bulk"
+	 * path can not be temporary maintained.
+	 */
 	for (; head; head = next) {
 		unsigned long offset = (unsigned long)head->func;
 
 		next = head->next;
-		// Potentially optimize with kfree_bulk in future.
 		debug_rcu_head_unqueue(head);
 		rcu_lock_acquire(&rcu_callback_map);
 		trace_rcu_invoke_kfree_callback(rcu_state.name, head, offset);
 
-		if (!WARN_ON_ONCE(!__is_kfree_rcu_offset(offset))) {
-			/* Could be optimized with kfree_bulk() in future. */
+		if (!WARN_ON_ONCE(!__is_kfree_rcu_offset(offset)))
 			kfree((void *)head - offset);
-		}
 
 		rcu_lock_release(&rcu_callback_map);
 		cond_resched_tasks_rcu_qs();
@@ -2774,26 +2893,48 @@ static void kfree_rcu_work(struct work_struct *work)
  */
 static inline bool queue_kfree_rcu_work(struct kfree_rcu_cpu *krcp)
 {
+	struct kfree_rcu_cpu_work *krwp;
+	bool queued = false;
 	int i;
-	struct kfree_rcu_cpu_work *krwp = NULL;
 
 	lockdep_assert_held(&krcp->lock);
-	for (i = 0; i < KFREE_N_BATCHES; i++)
-		if (!krcp->krw_arr[i].head_free) {
-			krwp = &(krcp->krw_arr[i]);
-			break;
-		}
 
-	// If a previous RCU batch is in progress, we cannot immediately
-	// queue another one, so return false to tell caller to retry.
-	if (!krwp)
-		return false;
+	for (i = 0; i < KFREE_N_BATCHES; i++) {
+		krwp = &(krcp->krw_arr[i]);
 
-	krwp->head_free = krcp->head;
-	krcp->head = NULL;
-	INIT_RCU_WORK(&krwp->rcu_work, kfree_rcu_work);
-	queue_rcu_work(system_wq, &krwp->rcu_work);
-	return true;
+		/*
+		 * Try to detach bhead or head and attach it over any
+		 * available corresponding free channel. It can be that
+		 * a previous RCU batch is in progress, it means that
+		 * immediately to queue another one is not possible so
+		 * return false to tell caller to retry.
+		 */
+		if ((krcp->bhead && !krwp->bhead_free) ||
+				(krcp->head && !krwp->head_free)) {
+			/* Channel 1. */
+			if (!krwp->bhead_free) {
+				krwp->bhead_free = krcp->bhead;
+				krcp->bhead = NULL;
+			}
+
+			/* Channel 2. */
+			if (!krwp->head_free) {
+				krwp->head_free = krcp->head;
+				krcp->head = NULL;
+			}
+
+			/*
+			 * One work is per one batch, so there are two "free channels",
+			 * "bhead_free" and "head_free" the batch can handle. It can be
+			 * that the work is in the pending state when two channels have
+			 * been detached following each other, one by one.
+			 */
+			queue_rcu_work(system_wq, &krwp->rcu_work);
+			queued = true;
+		}
+	}
+
+	return queued;
 }
 
 static inline void kfree_rcu_drain_unlock(struct kfree_rcu_cpu *krcp,
@@ -2830,19 +2971,65 @@ static void kfree_rcu_monitor(struct work_struct *work)
 		spin_unlock_irqrestore(&krcp->lock, flags);
 }
 
+static inline bool
+kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
+	struct rcu_head *head, rcu_callback_t func)
+{
+	struct kfree_rcu_bulk_data *bnode;
+
+	if (unlikely(!krcp->initialized))
+		return false;
+
+	lockdep_assert_held(&krcp->lock);
+
+	/* Check if a new block is required. */
+	if (!krcp->bhead ||
+			krcp->bhead->nr_records == KFREE_BULK_MAX_ENTR) {
+		bnode = xchg(&krcp->bcached, NULL);
+		if (!bnode) {
+			WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE);
+
+			bnode = (struct kfree_rcu_bulk_data *)
+				__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
+		}
+
+		/* Switch to emergency path. */
+		if (unlikely(!bnode))
+			return false;
+
+		/* Initialize the new block. */
+		bnode->nr_records = 0;
+		bnode->next = krcp->bhead;
+		bnode->head_free_debug = NULL;
+
+		/* Attach it to the head. */
+		krcp->bhead = bnode;
+	}
+
+#ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
+	head->func = func;
+	head->next = krcp->bhead->head_free_debug;
+	krcp->bhead->head_free_debug = head;
+#endif
+
+	/* Finally insert. */
+	krcp->bhead->records[krcp->bhead->nr_records++] =
+		(void *) head - (unsigned long) func;
+
+	return true;
+}
+
 /*
- * Queue a request for lazy invocation of kfree() after a grace period.
+ * Queue a request for lazy invocation of kfree_bulk()/kfree() after a grace
+ * period. Please note there are two paths are maintained, one is the main one
+ * that uses kfree_bulk() interface and second one is emergency one, that is
+ * used only when the main path can not be maintained temporary, due to memory
+ * pressure.
  *
  * Each kfree_call_rcu() request is added to a batch. The batch will be drained
- * every KFREE_DRAIN_JIFFIES number of jiffies. All the objects in the batch
- * will be kfree'd in workqueue context. This allows us to:
- *
- * 1.	Batch requests together to reduce the number of grace periods during
- *	heavy kfree_rcu() load.
- *
- * 2.	It makes it possible to use kfree_bulk() on a large number of
- *	kfree_rcu() requests thus reducing cache misses and the per-object
- *	overhead of kfree().
+ * every KFREE_DRAIN_JIFFIES number of jiffies. All the objects in the batch will
+ * be free'd in workqueue context. This allows us to: batch requests together to
+ * reduce the number of grace periods during heavy kfree_rcu() load.
  */
 void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
 {
@@ -2861,9 +3048,16 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
 			  __func__, head);
 		goto unlock_return;
 	}
-	head->func = func;
-	head->next = krcp->head;
-	krcp->head = head;
+
+	/*
+	 * Under high memory pressure GFP_NOWAIT can fail,
+	 * in that case the emergency path is maintained.
+	 */
+	if (unlikely(!kfree_call_rcu_add_ptr_to_bulk(krcp, head, func))) {
+		head->func = func;
+		head->next = krcp->head;
+		krcp->head = head;
+	}
 
 	// Set timer to drain after KFREE_DRAIN_JIFFIES.
 	if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
@@ -3075,24 +3269,32 @@ static void rcu_barrier_trace(const char *s, int cpu, unsigned long done)
 /*
  * RCU callback function for rcu_barrier().  If we are last, wake
  * up the task executing rcu_barrier().
+ *
+ * Note that the value of rcu_state.barrier_sequence must be captured
+ * before the atomic_dec_and_test().  Otherwise, if this CPU is not last,
+ * other CPUs might count the value down to zero before this CPU gets
+ * around to invoking rcu_barrier_trace(), which might result in bogus
+ * data from the next instance of rcu_barrier().
  */
 static void rcu_barrier_callback(struct rcu_head *rhp)
 {
+	unsigned long __maybe_unused s = rcu_state.barrier_sequence;
+
 	if (atomic_dec_and_test(&rcu_state.barrier_cpu_count)) {
-		rcu_barrier_trace(TPS("LastCB"), -1,
-				  rcu_state.barrier_sequence);
+		rcu_barrier_trace(TPS("LastCB"), -1, s);
 		complete(&rcu_state.barrier_completion);
 	} else {
-		rcu_barrier_trace(TPS("CB"), -1, rcu_state.barrier_sequence);
+		rcu_barrier_trace(TPS("CB"), -1, s);
 	}
 }
 
 /*
  * Called with preemption disabled, and from cross-cpu IRQ context.
  */
-static void rcu_barrier_func(void *unused)
+static void rcu_barrier_func(void *cpu_in)
 {
-	struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
+	uintptr_t cpu = (uintptr_t)cpu_in;
+	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 
 	rcu_barrier_trace(TPS("IRQ"), -1, rcu_state.barrier_sequence);
 	rdp->barrier_head.func = rcu_barrier_callback;
@@ -3119,7 +3321,7 @@ static void rcu_barrier_func(void *unused)
  */
 void rcu_barrier(void)
 {
-	int cpu;
+	uintptr_t cpu;
 	struct rcu_data *rdp;
 	unsigned long s = rcu_seq_snap(&rcu_state.barrier_sequence);
 
@@ -3142,13 +3344,14 @@ void rcu_barrier(void)
 	rcu_barrier_trace(TPS("Inc1"), -1, rcu_state.barrier_sequence);
 
 	/*
-	 * Initialize the count to one rather than to zero in order to
-	 * avoid a too-soon return to zero in case of a short grace period
-	 * (or preemption of this task).  Exclude CPU-hotplug operations
-	 * to ensure that no offline CPU has callbacks queued.
+	 * Initialize the count to two rather than to zero in order
+	 * to avoid a too-soon return to zero in case of an immediate
+	 * invocation of the just-enqueued callback (or preemption of
+	 * this task).  Exclude CPU-hotplug operations to ensure that no
+	 * offline non-offloaded CPU has callbacks queued.
 	 */
 	init_completion(&rcu_state.barrier_completion);
-	atomic_set(&rcu_state.barrier_cpu_count, 1);
+	atomic_set(&rcu_state.barrier_cpu_count, 2);
 	get_online_cpus();
 
 	/*
@@ -3158,13 +3361,23 @@ void rcu_barrier(void)
 	 */
 	for_each_possible_cpu(cpu) {
 		rdp = per_cpu_ptr(&rcu_data, cpu);
-		if (!cpu_online(cpu) &&
+		if (cpu_is_offline(cpu) &&
 		    !rcu_segcblist_is_offloaded(&rdp->cblist))
 			continue;
-		if (rcu_segcblist_n_cbs(&rdp->cblist)) {
+		if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_online(cpu)) {
 			rcu_barrier_trace(TPS("OnlineQ"), cpu,
 					  rcu_state.barrier_sequence);
-			smp_call_function_single(cpu, rcu_barrier_func, NULL, 1);
+			smp_call_function_single(cpu, rcu_barrier_func, (void *)cpu, 1);
+		} else if (rcu_segcblist_n_cbs(&rdp->cblist) &&
+			   cpu_is_offline(cpu)) {
+			rcu_barrier_trace(TPS("OfflineNoCBQ"), cpu,
+					  rcu_state.barrier_sequence);
+			local_irq_disable();
+			rcu_barrier_func((void *)cpu);
+			local_irq_enable();
+		} else if (cpu_is_offline(cpu)) {
+			rcu_barrier_trace(TPS("OfflineNoCBNoQ"), cpu,
+					  rcu_state.barrier_sequence);
 		} else {
 			rcu_barrier_trace(TPS("OnlineNQ"), cpu,
 					  rcu_state.barrier_sequence);
@@ -3176,7 +3389,7 @@ void rcu_barrier(void)
 	 * Now that we have an rcu_barrier_callback() callback on each
 	 * CPU, and thus each counted, remove the initial count.
 	 */
-	if (atomic_dec_and_test(&rcu_state.barrier_cpu_count))
+	if (atomic_sub_and_test(2, &rcu_state.barrier_cpu_count))
 		complete(&rcu_state.barrier_completion);
 
 	/* Wait for all rcu_barrier_callback() callbacks to be invoked. */
@@ -3275,12 +3488,12 @@ int rcutree_prepare_cpu(unsigned int cpu)
 	rnp = rdp->mynode;
 	raw_spin_lock_rcu_node(rnp);		/* irqs already disabled. */
 	rdp->beenonline = true;	 /* We have now been online. */
-	rdp->gp_seq = rnp->gp_seq;
-	rdp->gp_seq_needed = rnp->gp_seq;
+	rdp->gp_seq = READ_ONCE(rnp->gp_seq);
+	rdp->gp_seq_needed = rdp->gp_seq;
 	rdp->cpu_no_qs.b.norm = true;
 	rdp->core_needs_qs = false;
 	rdp->rcu_iw_pending = false;
-	rdp->rcu_iw_gp_seq = rnp->gp_seq - 1;
+	rdp->rcu_iw_gp_seq = rdp->gp_seq - 1;
 	trace_rcu_grace_period(rcu_state.name, rdp->gp_seq, TPS("cpuonl"));
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	rcu_prepare_kthreads(cpu);
@@ -3378,7 +3591,7 @@ void rcu_cpu_starting(unsigned int cpu)
 	rnp = rdp->mynode;
 	mask = rdp->grpmask;
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
-	rnp->qsmaskinitnext |= mask;
+	WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);
 	oldmask = rnp->expmaskinitnext;
 	rnp->expmaskinitnext |= mask;
 	oldmask ^= rnp->expmaskinitnext;
@@ -3431,7 +3644,7 @@ void rcu_report_dead(unsigned int cpu)
 		rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	}
-	rnp->qsmaskinitnext &= ~mask;
+	WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext & ~mask);
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	raw_spin_unlock(&rcu_state.ofl_lock);
 
@@ -3545,7 +3758,10 @@ static int __init rcu_spawn_gp_kthread(void)
 	}
 	rnp = rcu_get_root();
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
-	rcu_state.gp_kthread = t;
+	WRITE_ONCE(rcu_state.gp_activity, jiffies);
+	WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
+	// Reset .gp_activity and .gp_req_activity before setting .gp_kthread.
+	smp_store_release(&rcu_state.gp_kthread, t);  /* ^^^ */
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	wake_up_process(t);
 	rcu_spawn_nocb_kthreads();
@@ -3769,8 +3985,11 @@ static void __init kfree_rcu_batch_init(void)
 		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
 
 		spin_lock_init(&krcp->lock);
-		for (i = 0; i < KFREE_N_BATCHES; i++)
+		for (i = 0; i < KFREE_N_BATCHES; i++) {
+			INIT_RCU_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work);
 			krcp->krw_arr[i].krcp = krcp;
+		}
+
 		INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
 		krcp->initialized = true;
 	}
@@ -3809,6 +4028,13 @@ void __init rcu_init(void)
 	rcu_par_gp_wq = alloc_workqueue("rcu_par_gp", WQ_MEM_RECLAIM, 0);
 	WARN_ON(!rcu_par_gp_wq);
 	srcu_init();
+
+	/* Fill in default value for rcutree.qovld boot parameter. */
+	/* -After- the rcu_node ->lock fields are initialized! */
+	if (qovld < 0)
+		qovld_calc = DEFAULT_RCU_QOVLD_MULT * qhimark;
+	else
+		qovld_calc = qovld;
 }
 
 #include "tree_stall.h"

kernel/rcu/tree.h

@@ -68,6 +68,8 @@ struct rcu_node {
 				/* Online CPUs for next expedited GP. */
 				/*  Any CPU that has ever been online will */
 				/*  have its bit set. */
+	unsigned long cbovldmask;
+				/* CPUs experiencing callback overload. */
 	unsigned long ffmask;	/* Fully functional CPUs. */
 	unsigned long grpmask;	/* Mask to apply to parent qsmask. */
 				/*  Only one bit will be set in this mask. */
@@ -321,6 +323,8 @@ struct rcu_state {
 	atomic_t expedited_need_qs;		/* # CPUs left to check in. */
 	struct swait_queue_head expedited_wq;	/* Wait for check-ins. */
 	int ncpus_snap;				/* # CPUs seen last time. */
+	u8 cbovld;				/* Callback overload now? */
+	u8 cbovldnext;				/* ^        ^  next time? */
 
 	unsigned long jiffies_force_qs;		/* Time at which to invoke */
 						/*  force_quiescent_state(). */

kernel/rcu/tree_exp.h

@@ -314,7 +314,7 @@ static bool exp_funnel_lock(unsigned long s)
 				   sync_exp_work_done(s));
 			return true;
 		}
-		rnp->exp_seq_rq = s; /* Followers can wait on us. */
+		WRITE_ONCE(rnp->exp_seq_rq, s); /* Followers can wait on us. */
 		spin_unlock(&rnp->exp_lock);
 		trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
 					  rnp->grplo, rnp->grphi, TPS("nxtlvl"));
@@ -485,6 +485,7 @@ static bool synchronize_rcu_expedited_wait_once(long tlimit)
 static void synchronize_rcu_expedited_wait(void)
 {
 	int cpu;
+	unsigned long j;
 	unsigned long jiffies_stall;
 	unsigned long jiffies_start;
 	unsigned long mask;
@@ -496,7 +497,7 @@ static void synchronize_rcu_expedited_wait(void)
 	trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait"));
 	jiffies_stall = rcu_jiffies_till_stall_check();
 	jiffies_start = jiffies;
-	if (IS_ENABLED(CONFIG_NO_HZ_FULL)) {
+	if (tick_nohz_full_enabled() && rcu_inkernel_boot_has_ended()) {
 		if (synchronize_rcu_expedited_wait_once(1))
 			return;
 		rcu_for_each_leaf_node(rnp) {
@@ -508,12 +509,16 @@ static void synchronize_rcu_expedited_wait(void)
 				tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
 			}
 		}
+		j = READ_ONCE(jiffies_till_first_fqs);
+		if (synchronize_rcu_expedited_wait_once(j + HZ))
+			return;
+		WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT));
 	}
 
 	for (;;) {
 		if (synchronize_rcu_expedited_wait_once(jiffies_stall))
 			return;
-		if (rcu_cpu_stall_suppress)
+		if (rcu_stall_is_suppressed())
 			continue;
 		panic_on_rcu_stall();
 		pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {",
@@ -589,7 +594,7 @@ static void rcu_exp_wait_wake(unsigned long s)
 			spin_lock(&rnp->exp_lock);
 			/* Recheck, avoid hang in case someone just arrived. */
 			if (ULONG_CMP_LT(rnp->exp_seq_rq, s))
-				rnp->exp_seq_rq = s;
+				WRITE_ONCE(rnp->exp_seq_rq, s);
 			spin_unlock(&rnp->exp_lock);
 		}
 		smp_mb(); /* All above changes before wakeup. */

kernel/rcu/tree_plugin.h

@@ -56,6 +56,8 @@ static void __init rcu_bootup_announce_oddness(void)
 		pr_info("\tBoot-time adjustment of callback high-water mark to %ld.\n", qhimark);
 	if (qlowmark != DEFAULT_RCU_QLOMARK)
 		pr_info("\tBoot-time adjustment of callback low-water mark to %ld.\n", qlowmark);
+	if (qovld != DEFAULT_RCU_QOVLD)
+		pr_info("\tBoot-time adjustment of callback overload level to %ld.\n", qovld);
 	if (jiffies_till_first_fqs != ULONG_MAX)
 		pr_info("\tBoot-time adjustment of first FQS scan delay to %ld jiffies.\n", jiffies_till_first_fqs);
 	if (jiffies_till_next_fqs != ULONG_MAX)
@@ -753,7 +755,7 @@ dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
 	raw_lockdep_assert_held_rcu_node(rnp);
 	pr_info("%s: grp: %d-%d level: %d ->gp_seq %ld ->completedqs %ld\n",
 		__func__, rnp->grplo, rnp->grphi, rnp->level,
-		(long)rnp->gp_seq, (long)rnp->completedqs);
+		(long)READ_ONCE(rnp->gp_seq), (long)rnp->completedqs);
 	for (rnp1 = rnp; rnp1; rnp1 = rnp1->parent)
 		pr_info("%s: %d:%d ->qsmask %#lx ->qsmaskinit %#lx ->qsmaskinitnext %#lx\n",
 			__func__, rnp1->grplo, rnp1->grphi, rnp1->qsmask, rnp1->qsmaskinit, rnp1->qsmaskinitnext);
@@ -1032,18 +1034,18 @@ static int rcu_boost_kthread(void *arg)
 
 	trace_rcu_utilization(TPS("Start boost kthread@init"));
 	for (;;) {
-		rnp->boost_kthread_status = RCU_KTHREAD_WAITING;
+		WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_WAITING);
 		trace_rcu_utilization(TPS("End boost kthread@rcu_wait"));
 		rcu_wait(rnp->boost_tasks || rnp->exp_tasks);
 		trace_rcu_utilization(TPS("Start boost kthread@rcu_wait"));
-		rnp->boost_kthread_status = RCU_KTHREAD_RUNNING;
+		WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_RUNNING);
 		more2boost = rcu_boost(rnp);
 		if (more2boost)
 			spincnt++;
 		else
 			spincnt = 0;
 		if (spincnt > 10) {
-			rnp->boost_kthread_status = RCU_KTHREAD_YIELDING;
+			WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_YIELDING);
 			trace_rcu_utilization(TPS("End boost kthread@rcu_yield"));
 			schedule_timeout_interruptible(2);
 			trace_rcu_utilization(TPS("Start boost kthread@rcu_yield"));
@@ -1077,12 +1079,12 @@ static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
 	    (rnp->gp_tasks != NULL &&
 	     rnp->boost_tasks == NULL &&
 	     rnp->qsmask == 0 &&
-	     ULONG_CMP_GE(jiffies, rnp->boost_time))) {
+	     (ULONG_CMP_GE(jiffies, rnp->boost_time) || rcu_state.cbovld))) {
 		if (rnp->exp_tasks == NULL)
 			rnp->boost_tasks = rnp->gp_tasks;
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 		rcu_wake_cond(rnp->boost_kthread_task,
-			      rnp->boost_kthread_status);
+			      READ_ONCE(rnp->boost_kthread_status));
 	} else {
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	}
@@ -1486,6 +1488,7 @@ module_param(nocb_nobypass_lim_per_jiffy, int, 0);
  * flag the contention.
  */
 static void rcu_nocb_bypass_lock(struct rcu_data *rdp)
+	__acquires(&rdp->nocb_bypass_lock)
 {
 	lockdep_assert_irqs_disabled();
 	if (raw_spin_trylock(&rdp->nocb_bypass_lock))
@@ -1529,6 +1532,7 @@ static bool rcu_nocb_bypass_trylock(struct rcu_data *rdp)
  * Release the specified rcu_data structure's ->nocb_bypass_lock.
  */
 static void rcu_nocb_bypass_unlock(struct rcu_data *rdp)
+	__releases(&rdp->nocb_bypass_lock)
 {
 	lockdep_assert_irqs_disabled();
 	raw_spin_unlock(&rdp->nocb_bypass_lock);
@@ -1577,8 +1581,7 @@ static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
 static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp)
 {
 	lockdep_assert_irqs_disabled();
-	if (rcu_segcblist_is_offloaded(&rdp->cblist) &&
-	    cpu_online(rdp->cpu))
+	if (rcu_segcblist_is_offloaded(&rdp->cblist))
 		lockdep_assert_held(&rdp->nocb_lock);
 }
 
@@ -1930,6 +1933,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
 	struct rcu_data *rdp;
 	struct rcu_node *rnp;
 	unsigned long wait_gp_seq = 0; // Suppress "use uninitialized" warning.
+	bool wasempty = false;
 
 	/*
 	 * Each pass through the following loop checks for CBs and for the
@@ -1969,10 +1973,13 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
 		     rcu_seq_done(&rnp->gp_seq, cur_gp_seq))) {
 			raw_spin_lock_rcu_node(rnp); /* irqs disabled. */
 			needwake_gp = rcu_advance_cbs(rnp, rdp);
+			wasempty = rcu_segcblist_restempty(&rdp->cblist,
+							   RCU_NEXT_READY_TAIL);
 			raw_spin_unlock_rcu_node(rnp); /* irqs disabled. */
 		}
 		// Need to wait on some grace period?
-		WARN_ON_ONCE(!rcu_segcblist_restempty(&rdp->cblist,
+		WARN_ON_ONCE(wasempty &&
+			     !rcu_segcblist_restempty(&rdp->cblist,
 						      RCU_NEXT_READY_TAIL));
 		if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq)) {
 			if (!needwait_gp ||

kernel/rcu/tree_stall.h

@@ -102,7 +102,7 @@ static void record_gp_stall_check_time(void)
 	unsigned long j = jiffies;
 	unsigned long j1;
 
-	rcu_state.gp_start = j;
+	WRITE_ONCE(rcu_state.gp_start, j);
 	j1 = rcu_jiffies_till_stall_check();
 	/* Record ->gp_start before ->jiffies_stall. */
 	smp_store_release(&rcu_state.jiffies_stall, j + j1); /* ^^^ */
@@ -383,7 +383,7 @@ static void print_other_cpu_stall(unsigned long gp_seq)
 
 	/* Kick and suppress, if so configured. */
 	rcu_stall_kick_kthreads();
-	if (rcu_cpu_stall_suppress)
+	if (rcu_stall_is_suppressed())
 		return;
 
 	/*
@@ -452,7 +452,7 @@ static void print_cpu_stall(void)
 
 	/* Kick and suppress, if so configured. */
 	rcu_stall_kick_kthreads();
-	if (rcu_cpu_stall_suppress)
+	if (rcu_stall_is_suppressed())
 		return;
 
 	/*
@@ -504,7 +504,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
 	unsigned long js;
 	struct rcu_node *rnp;
 
-	if ((rcu_cpu_stall_suppress && !rcu_kick_kthreads) ||
+	if ((rcu_stall_is_suppressed() && !rcu_kick_kthreads) ||
 	    !rcu_gp_in_progress())
 		return;
 	rcu_stall_kick_kthreads();
@@ -578,6 +578,7 @@ void show_rcu_gp_kthreads(void)
 	unsigned long jw;
 	struct rcu_data *rdp;
 	struct rcu_node *rnp;
+	struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
 
 	j = jiffies;
 	ja = j - READ_ONCE(rcu_state.gp_activity);
@@ -585,28 +586,28 @@ void show_rcu_gp_kthreads(void)
 	jw = j - READ_ONCE(rcu_state.gp_wake_time);
 	pr_info("%s: wait state: %s(%d) ->state: %#lx delta ->gp_activity %lu ->gp_req_activity %lu ->gp_wake_time %lu ->gp_wake_seq %ld ->gp_seq %ld ->gp_seq_needed %ld ->gp_flags %#x\n",
 		rcu_state.name, gp_state_getname(rcu_state.gp_state),
-		rcu_state.gp_state,
-		rcu_state.gp_kthread ? rcu_state.gp_kthread->state : 0x1ffffL,
+		rcu_state.gp_state, t ? t->state : 0x1ffffL,
 		ja, jr, jw, (long)READ_ONCE(rcu_state.gp_wake_seq),
 		(long)READ_ONCE(rcu_state.gp_seq),
 		(long)READ_ONCE(rcu_get_root()->gp_seq_needed),
 		READ_ONCE(rcu_state.gp_flags));
 	rcu_for_each_node_breadth_first(rnp) {
-		if (ULONG_CMP_GE(rcu_state.gp_seq, rnp->gp_seq_needed))
+		if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq),
+				 READ_ONCE(rnp->gp_seq_needed)))
 			continue;
 		pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld\n",
-			rnp->grplo, rnp->grphi, (long)rnp->gp_seq,
-			(long)rnp->gp_seq_needed);
+			rnp->grplo, rnp->grphi, (long)READ_ONCE(rnp->gp_seq),
+			(long)READ_ONCE(rnp->gp_seq_needed));
 		if (!rcu_is_leaf_node(rnp))
 			continue;
 		for_each_leaf_node_possible_cpu(rnp, cpu) {
 			rdp = per_cpu_ptr(&rcu_data, cpu);
-			if (rdp->gpwrap ||
-			    ULONG_CMP_GE(rcu_state.gp_seq,
-					 rdp->gp_seq_needed))
+			if (READ_ONCE(rdp->gpwrap) ||
+			    ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq),
+					 READ_ONCE(rdp->gp_seq_needed)))
 				continue;
 			pr_info("\tcpu %d ->gp_seq_needed %ld\n",
-				cpu, (long)rdp->gp_seq_needed);
+				cpu, (long)READ_ONCE(rdp->gp_seq_needed));
 		}
 	}
 	for_each_possible_cpu(cpu) {
@@ -631,7 +632,9 @@ static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
 	static atomic_t warned = ATOMIC_INIT(0);
 
 	if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
-	    ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed))
+	    ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
+			 READ_ONCE(rnp_root->gp_seq_needed)) ||
+	    !smp_load_acquire(&rcu_state.gp_kthread)) // Get stable kthread.
 		return;
 	j = jiffies; /* Expensive access, and in common case don't get here. */
 	if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
@@ -642,7 +645,8 @@ static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	j = jiffies;
 	if (rcu_gp_in_progress() ||
-	    ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed) ||
+	    ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
+			 READ_ONCE(rnp_root->gp_seq_needed)) ||
 	    time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
 	    time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
 	    atomic_read(&warned)) {
@@ -655,9 +659,10 @@ static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
 		raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
 	j = jiffies;
 	if (rcu_gp_in_progress() ||
-	    ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed) ||
-	    time_before(j, rcu_state.gp_req_activity + gpssdelay) ||
-	    time_before(j, rcu_state.gp_activity + gpssdelay) ||
+	    ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
+			 READ_ONCE(rnp_root->gp_seq_needed)) ||
+	    time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
+	    time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
 	    atomic_xchg(&warned, 1)) {
 		if (rnp_root != rnp)
 			/* irqs remain disabled. */

kernel/rcu/update.c

@@ -183,6 +183,8 @@ void rcu_unexpedite_gp(void)
 }
 EXPORT_SYMBOL_GPL(rcu_unexpedite_gp);
 
+static bool rcu_boot_ended __read_mostly;
+
 /*
  * Inform RCU of the end of the in-kernel boot sequence.
  */
@@ -191,8 +193,18 @@ void rcu_end_inkernel_boot(void)
 	rcu_unexpedite_gp();
 	if (rcu_normal_after_boot)
 		WRITE_ONCE(rcu_normal, 1);
+	rcu_boot_ended = 1;
 }
 
+/*
+ * Let rcutorture know when it is OK to turn it up to eleven.
+ */
+bool rcu_inkernel_boot_has_ended(void)
+{
+	return rcu_boot_ended;
+}
+EXPORT_SYMBOL_GPL(rcu_inkernel_boot_has_ended);
+
 #endif /* #ifndef CONFIG_TINY_RCU */
 
 /*
@@ -464,13 +476,19 @@ EXPORT_SYMBOL_GPL(rcutorture_sched_setaffinity);
 #ifdef CONFIG_RCU_STALL_COMMON
 int rcu_cpu_stall_ftrace_dump __read_mostly;
 module_param(rcu_cpu_stall_ftrace_dump, int, 0644);
-int rcu_cpu_stall_suppress __read_mostly; /* 1 = suppress stall warnings. */
+int rcu_cpu_stall_suppress __read_mostly; // !0 = suppress stall warnings.
 EXPORT_SYMBOL_GPL(rcu_cpu_stall_suppress);
 module_param(rcu_cpu_stall_suppress, int, 0644);
 int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT;
 module_param(rcu_cpu_stall_timeout, int, 0644);
 #endif /* #ifdef CONFIG_RCU_STALL_COMMON */
 
+// Suppress boot-time RCU CPU stall warnings and rcutorture writer stall
+// warnings.  Also used by rcutorture even if stall warnings are excluded.
+int rcu_cpu_stall_suppress_at_boot __read_mostly; // !0 = suppress boot stalls.
+EXPORT_SYMBOL_GPL(rcu_cpu_stall_suppress_at_boot);
+module_param(rcu_cpu_stall_suppress_at_boot, int, 0444);
+
 #ifdef CONFIG_TASKS_RCU
 
 /*
@@ -528,7 +546,7 @@ void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
 	rhp->func = func;
 	raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
 	needwake = !rcu_tasks_cbs_head;
-	*rcu_tasks_cbs_tail = rhp;
+	WRITE_ONCE(*rcu_tasks_cbs_tail, rhp);
 	rcu_tasks_cbs_tail = &rhp->next;
 	raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
 	/* We can't create the thread unless interrupts are enabled. */
@@ -658,7 +676,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 		/* If there were none, wait a bit and start over. */
 		if (!list) {
 			wait_event_interruptible(rcu_tasks_cbs_wq,
-						 rcu_tasks_cbs_head);
+						 READ_ONCE(rcu_tasks_cbs_head));
 			if (!rcu_tasks_cbs_head) {
 				WARN_ON(signal_pending(current));
 				schedule_timeout_interruptible(HZ/10);
@@ -801,7 +819,7 @@ static int __init rcu_spawn_tasks_kthread(void)
 core_initcall(rcu_spawn_tasks_kthread);
 
 /* Do the srcu_read_lock() for the above synchronize_srcu().  */
-void exit_tasks_rcu_start(void)
+void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu)
 {
 	preempt_disable();
 	current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu);
@@ -809,7 +827,7 @@ void exit_tasks_rcu_start(void)
 }
 
 /* Do the srcu_read_unlock() for the above synchronize_srcu().  */
-void exit_tasks_rcu_finish(void)
+void exit_tasks_rcu_finish(void) __releases(&tasks_rcu_exit_srcu)
 {
 	preempt_disable();
 	__srcu_read_unlock(&tasks_rcu_exit_srcu, current->rcu_tasks_idx);

kernel/time/timer.c

@@ -944,6 +944,7 @@ static struct timer_base *lock_timer_base(struct timer_list *timer,
 
 #define MOD_TIMER_PENDING_ONLY		0x01
 #define MOD_TIMER_REDUCE		0x02
+#define MOD_TIMER_NOTPENDING		0x04
 
 static inline int
 __mod_timer(struct timer_list *timer, unsigned long expires, unsigned int options)
@@ -960,7 +961,7 @@ __mod_timer(struct timer_list *timer, unsigned long expires, unsigned int option
 	 * the timer is re-modified to have the same timeout or ends up in the
 	 * same array bucket then just return:
 	 */
-	if (timer_pending(timer)) {
+	if (!(options & MOD_TIMER_NOTPENDING) && timer_pending(timer)) {
 		/*
 		 * The downside of this optimization is that it can result in
 		 * larger granularity than you would get from adding a new
@@ -1133,7 +1134,7 @@ EXPORT_SYMBOL(timer_reduce);
 void add_timer(struct timer_list *timer)
 {
 	BUG_ON(timer_pending(timer));
-	mod_timer(timer, timer->expires);
+	__mod_timer(timer, timer->expires, MOD_TIMER_NOTPENDING);
 }
 EXPORT_SYMBOL(add_timer);
 
@@ -1891,7 +1892,7 @@ signed long __sched schedule_timeout(signed long timeout)
 
 	timer.task = current;
 	timer_setup_on_stack(&timer.timer, process_timeout, 0);
-	__mod_timer(&timer.timer, expire, 0);
+	__mod_timer(&timer.timer, expire, MOD_TIMER_NOTPENDING);
 	schedule();
 	del_singleshot_timer_sync(&timer.timer);
 

kernel/torture.c

@@ -42,6 +42,9 @@
 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("Paul E. McKenney <paulmck@linux.ibm.com>");
 
+static bool disable_onoff_at_boot;
+module_param(disable_onoff_at_boot, bool, 0444);
+
 static char *torture_type;
 static int verbose;
 
@@ -84,6 +87,7 @@ bool torture_offline(int cpu, long *n_offl_attempts, long *n_offl_successes,
 {
 	unsigned long delta;
 	int ret;
+	char *s;
 	unsigned long starttime;
 
 	if (!cpu_online(cpu) || !cpu_is_hotpluggable(cpu))
@@ -99,10 +103,16 @@ bool torture_offline(int cpu, long *n_offl_attempts, long *n_offl_successes,
 	(*n_offl_attempts)++;
 	ret = cpu_down(cpu);
 	if (ret) {
+		s = "";
+		if (!rcu_inkernel_boot_has_ended() && ret == -EBUSY) {
+			// PCI probe frequently disables hotplug during boot.
+			(*n_offl_attempts)--;
+			s = " (-EBUSY forgiven during boot)";
+		}
 		if (verbose)
 			pr_alert("%s" TORTURE_FLAG
-				 "torture_onoff task: offline %d failed: errno %d\n",
-				 torture_type, cpu, ret);
+				 "torture_onoff task: offline %d failed%s: errno %d\n",
+				 torture_type, cpu, s, ret);
 	} else {
 		if (verbose > 1)
 			pr_alert("%s" TORTURE_FLAG
@@ -137,6 +147,7 @@ bool torture_online(int cpu, long *n_onl_attempts, long *n_onl_successes,
 {
 	unsigned long delta;
 	int ret;
+	char *s;
 	unsigned long starttime;
 
 	if (cpu_online(cpu) || !cpu_is_hotpluggable(cpu))
@@ -150,10 +161,16 @@ bool torture_online(int cpu, long *n_onl_attempts, long *n_onl_successes,
 	(*n_onl_attempts)++;
 	ret = cpu_up(cpu);
 	if (ret) {
+		s = "";
+		if (!rcu_inkernel_boot_has_ended() && ret == -EBUSY) {
+			// PCI probe frequently disables hotplug during boot.
+			(*n_onl_attempts)--;
+			s = " (-EBUSY forgiven during boot)";
+		}
 		if (verbose)
 			pr_alert("%s" TORTURE_FLAG
-				 "torture_onoff task: online %d failed: errno %d\n",
-				 torture_type, cpu, ret);
+				 "torture_onoff task: online %d failed%s: errno %d\n",
+				 torture_type, cpu, s, ret);
 	} else {
 		if (verbose > 1)
 			pr_alert("%s" TORTURE_FLAG
@@ -215,6 +232,10 @@ torture_onoff(void *arg)
 		VERBOSE_TOROUT_STRING("torture_onoff end holdoff");
 	}
 	while (!torture_must_stop()) {
+		if (disable_onoff_at_boot && !rcu_inkernel_boot_has_ended()) {
+			schedule_timeout_interruptible(HZ / 10);
+			continue;
+		}
 		cpu = (torture_random(&rand) >> 4) % (maxcpu + 1);
 		if (!torture_offline(cpu,
 				     &n_offline_attempts, &n_offline_successes,

tools/testing/selftests/rcutorture/bin/functions.sh

@@ -12,7 +12,7 @@
 # Returns 1 if the specified boot-parameter string tells rcutorture to
 # test CPU-hotplug operations.
 bootparam_hotplug_cpu () {
-	echo "$1" | grep -q "rcutorture\.onoff_"
+	echo "$1" | grep -q "torture\.onoff_"
 }
 
 # checkarg --argname argtype $# arg mustmatch cannotmatch

tools/testing/selftests/rcutorture/bin/kvm-find-errors.sh

@@ -20,7 +20,9 @@
 rundir="${1}"
 if test -z "$rundir" -o ! -d "$rundir"
 then
+	echo Directory "$rundir" not found.
 	echo Usage: $0 directory
+	exit 1
 fi
 editor=${EDITOR-vi}
 

tools/testing/selftests/rcutorture/bin/kvm-recheck.sh

@@ -13,6 +13,9 @@
 #
 # Authors: Paul E. McKenney <paulmck@linux.ibm.com>
 
+T=/tmp/kvm-recheck.sh.$$
+trap 'rm -f $T' 0 2
+
 PATH=`pwd`/tools/testing/selftests/rcutorture/bin:$PATH; export PATH
 . functions.sh
 for rd in "$@"
@@ -68,4 +71,16 @@ do
 		fi
 	done
 done
-EDITOR=echo kvm-find-errors.sh "${@: -1}" > /dev/null 2>&1
+EDITOR=echo kvm-find-errors.sh "${@: -1}" > $T 2>&1
+ret=$?
+builderrors="`tr ' ' '\012' < $T | grep -c '/Make.out.diags'`"
+if test "$builderrors" -gt 0
+then
+	echo $builderrors runs with build errors.
+fi
+runerrors="`tr ' ' '\012' < $T | grep -c '/console.log.diags'`"
+if test "$runerrors" -gt 0
+then
+	echo $runerrors runs with runtime errors.
+fi
+exit $ret

tools/testing/selftests/rcutorture/bin/kvm.sh

@@ -39,7 +39,7 @@ TORTURE_TRUST_MAKE=""
 resdir=""
 configs=""
 cpus=0
-ds=`date +%Y.%m.%d-%H:%M:%S`
+ds=`date +%Y.%m.%d-%H.%M.%S`
 jitter="-1"
 
 usage () {

tools/testing/selftests/rcutorture/configs/rcu/CFcommon

@@ -3,3 +3,5 @@ CONFIG_PRINTK_TIME=y
 CONFIG_HYPERVISOR_GUEST=y
 CONFIG_PARAVIRT=y
 CONFIG_KVM_GUEST=y
+CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=n
+CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY=n

tools/testing/selftests/rcutorture/configs/rcu/TREE10

+CONFIG_SMP=y
+CONFIG_NR_CPUS=100
+CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_VOLUNTARY=n
+CONFIG_PREEMPT=n
+#CHECK#CONFIG_TREE_RCU=y
+CONFIG_HZ_PERIODIC=n
+CONFIG_NO_HZ_IDLE=y
+CONFIG_NO_HZ_FULL=n
+CONFIG_RCU_FAST_NO_HZ=n
+CONFIG_RCU_TRACE=n
+CONFIG_RCU_NOCB_CPU=n
+CONFIG_DEBUG_LOCK_ALLOC=n
+CONFIG_PROVE_LOCKING=n
+#CHECK#CONFIG_PROVE_RCU=n
+CONFIG_DEBUG_OBJECTS=n
+CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
+CONFIG_RCU_EXPERT=n