Commit 984d74a7 ("sysrq: rcu-ify __handle_sysrq") replaced
spin_lock_irqsave() calls with rcu_read_lock() calls in sysrq. Since
rcu_read_lock() does not disable preemption, faulthandler_disabled() in
__do_page_fault() in x86/fault.c returns false. When the code later calls
might_sleep() in the pagefault handler, we get the following warning:

BUG: sleeping function called from invalid context at ../arch/x86/mm/fault.c:1187
in_atomic(): 0, irqs_disabled(): 0, pid: 4706, name: bash
Preemption disabled at:[<ffffffff81484339>] printk+0x48/0x4a

To fix this, we release the RCU read lock before we crash.

Tested this patch on linux 3.18 by booting off one of our boards.

Fixes: 984d74a7 ("sysrq: rcu-ify __handle_sysrq")
Signed-off-by: Ani Sinha <ani@arista.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Merge branches 'doc.2015.12.05a', 'exp.2015.12.07a', 'fixes.2015.12.07a', 'list.2015.12.04b' and 'torture.2015.12.05a' into HEAD

doc.2015.12.05a:  Documentation updates
exp.2015.12.07a:  Expedited grace-period updates
fixes.2015.12.07a:  Miscellaneous fixes
list.2015.12.04b:  Linked-list updates
torture.2015.12.05a:  Torture-test updates

Although list_for_each_entry_rcu() can in theory be used anywhere
preemption is disabled, it can result in calls to lockdep, which cannot
be used in certain constrained execution environments, such as exception
handlers that do not map the entire kernel into their address spaces.
This commit therefore adds list_entry_lockless() and
list_for_each_entry_lockless(), which never invoke lockdep and can
therefore safely be used from these constrained environments, but only
as long as those environments are non-preemptible (or items are never
deleted from the list).

Use synchronize_sched(), call_rcu_sched(), or synchronize_sched_expedited()
in updates for the needed grace periods.  Of course, if items are never
deleted from the list, there is no need to wait for grace periods.
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

The return value from rcu_gp_init() is always used as a bool, so
this commit makes it be a bool.
Reported-by: Iftekhar Ahmed <ahmedi@oregonstate.edu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

This patch removes a potential deadlock hazard by moving the
wake_up_process() in rcu_spawn_gp_kthread() out from under rnp->lock.
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

rcu_dereference_raw() calls indirectly rcu_read_lock_held() while
rcu_dereference_raw_notrace() does not so fix the comment about the latter.
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit replaces a local_irq_save()/local_irq_restore() pair with
a lockdep assertion that interrupts are already disabled.  This should
remove the corresponding overhead from the interrupt entry/exit fastpaths.

This change was inspired by the fact that Iftekhar Ahmed's mutation
testing showed that removing rcu_irq_enter()'s call to local_ird_restore()
had no effect, which might indicate that interrupts were always enabled
anyway.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

The cpu_needs_another_gp() function is currently of type int, but only
returns zero or one.  Bow to reality and make it be of type bool.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Now that the rcu_state structure's ->rda field is compile-time initialized,
there is no need to pass the per-CPU rcu_data structure into rcu_init_one().
This commit therefore eliminates this now-unused parameter.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

The rcu_expedited, rcu_normal, and rcu_normal_after_boot kernel boot
parameters are pointless in the case of TINY_RCU because in that case
synchronous grace periods, both expedited and normal, are no-ops.
However, these three symbols contribute several hundred bytes of bloat.
This commit therefore uses CPP directives to avoid compiling this code
in TINY_RCU kernels.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Currently, the console output files ("console.log") are placed in the
build directory initially, then copied to the results directory.
One problem with this is if a qemu refuses to die in a timely fashion
after a kernel hang, it will continue to write after the next qemu
starts up, resulting in confusing output from the old instance of
qemu.  This commit prevents such confusion by placing the console.log
files into the results directory to begin with, so that a given instance
of qemu is always writing only to its own console.log file.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Currently, the scripts print a list of warning/bug indicators from the
console.log file.  This works well if there are only a few warnings or
bugs, but can be quite annoying if there is a large number.  This commit
therefore prints a summary listing the number of each type of warning/bug
indicator, but only if there is at least one such indicator.  The full
list is stored in the results directory at console.log.diags, which
makes it easier to find the warning/bugs in the full console.log.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Currently, ->gp_state is printed as an integer, which slows debugging.
This commit therefore prints a symbolic name in addition to the integer.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Updated to fix relational operator called out by Dan Carpenter. ]
[ paulmck: More "const", as suggested by Josh Triplett. ]
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Currently, rcu_torture_writer_state is printed as an integer, which slows
debugging.  This commit therefore prints a symbolic name in addition to
the integer.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: More "const", as suggested by Josh Triplett. ]
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Commit d1ec4c34 ("rcu: Drop RCU_USER_QS in favor of NO_HZ_FULL") has
removed RCU_USER_QS from Kconfig file, so remove it from some documents
to avoid any confusion.
Signed-off-by: Yang Shi <yang.shi@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

The default test grace period of two minutes is insufficient in some
cases and excessive in others.  This commit therefore increases the
default to three minutes, but also adds a --shutdown-grace parameter
to allow the default to be overridden.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

This commit increases debug information in the case where the grace-period
kthread is being prevented from running by dumping that kthread's stack.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Split into prior commit and this commit, as suggested by
  Josh Triplett. ]
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Currently, if the RCU grace-period kthread has not yet been created,
in which case the starvation-check code will print zero for the state,
which maps to TASK_RUNNING.  This could clearly be quite confusing, so
this commit prints ~0, which does not map to any legal ->state value.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Currently, the scripts print "----Start batch" at the beginning of each
batch, which does serve as a good visual delimiter between batches.
Unfortunately, if there are a lot of batches, it is hard to quickly
estimate test runtime from the output of "--dryrun sched".  This commit
therefore adds a batch number, so that the beginning-of-batch output
looks like this "----Start batch 10" for the tenth batch.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

In commit 2ecf8101 ("Documentation/memory-barriers.txt: Add
needed ACCESS_ONCE() calls to memory-barriers.txt") the statement
"Q = P" was converted to "ACCESS_ONCE(Q) = P".  This should have
been "Q = ACCESS_ONCE(P)".  It later became "WRITE_ONCE(Q, P)".
This doesn't match the following text, which is "Q = LOAD P".
Change the statement to be "Q = READ_ONCE(P)".
Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Because RCU-sched expedited grace periods now use IPIs and interact
with rcu_read_unlock(), it is no longer sufficient to disable preemption
across RCU read-side critical sections that acquire and hold scheduler
locks.  It is now necessary to instead disable interrupts.  This commit
documents this change.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

The RCU requirements do not make it absolutely clear that the
memory-barrier requirements are not intended to replace the fundamental
requirement that all pre-existing RCU readers complete before a grace
period completes.  This commit therefore pulls the memory-barrier
requirements into a separate section and explicitly calls out the
relationship between the memory-barrier requirements and the fundamental
requirement.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit adds a second option for avoiding scheduler/RCU deadlocks,
namely that preemption be disabled across the entire RCU read-side
critical section in question.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit expands on RCU's composability by comparing it to that of
transactional memory and of locking.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit adds verbiage on boot and sysfs parameters that can be
used to control RCU CPU stall warnings, both to change the timeout
and to suppress these warnings entirely.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit records RCU's guarantee that the bottom bit of the rcu_head
structure's ->next field will remain zero for callbacks posted via
call_rcu(), but not necessarily for <tt>kfree_rcu()</tt> or some
possible future call_rcu_lazy() variant that might one day be created
for energy-efficiency purposese.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Updates URLs as suggested by Josh Triplett. ]

This commit adds RCU requirements as published in a 2015 LWN series.
Bringing these requirements in-tree allows them to be updated as changes
are discovered.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Updates to charset and URLs as suggested by Josh Triplett. ]

Code that does lockless emptiness testing of non-RCU lists is relying
on INIT_LIST_HEAD() to write the list head's ->next pointer atomically,
particularly when INIT_LIST_HEAD() is invoked from list_del_init().
This commit therefore adds WRITE_ONCE() to this function's pointer stores
that could affect the head's ->next pointer.
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

The list_splice_init_rcu() can be used as a stack onto which full lists
are pushed, but queue-like behavior is now needed by some security
policies.  This requires a list_splice_tail_init_rcu().

This commit therefore supplies a list_splice_tail_init_rcu() by
pulling code common it and to list_splice_init_rcu() into a new
__list_splice_init_rcu() function.  This new function is based on the
existing list_splice_init_rcu() implementation.
Signed-off-by: Petko Manolov <petkan@mip-labs.com>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

We need the scheduler's fastpaths to be, well, fast, and unnecessarily
disabling and re-enabling interrupts is not necessarily consistent with
this goal.  Especially given that there are regions of the scheduler that
already have interrupts disabled.

This commit therefore moves the call to rcu_note_context_switch()
to one of the interrupts-disabled regions of the scheduler, and
removes the now-redundant disabling and re-enabling of interrupts from
rcu_note_context_switch() and the functions it calls.
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Shift rcu_note_context_switch() to avoid deadlock, as suggested
  by Peter Zijlstra. ]

Currently, rcu_prepare_for_idle() checks for tick_nohz_active, even on
individual NOCBs CPUs, unless all CPUs are marked as NOCBs CPUs at build
time.  This check is pointless on NOCBs CPUs because they never have any
callbacks posted, given that all of their callbacks are handed off to the
corresponding rcuo kthread.  There is a check for individually designated
NOCBs CPUs, but it pointelessly follows the check for tick_nohz_active.

This commit therefore moves the check for individually designated NOCBs
CPUs up with the check for CONFIG_RCU_NOCB_CPU_ALL.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

This function no longer has #ifdefs, so this commit removes the
header comment calling them out.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Several releases have come and gone without the warning triggering,
so remove the lock-acquisition loop.  Retain the WARN_ON_ONCE()
out of sheer paranoia.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit applies an early-exit approach to rcu_sched_qs(), reducing
the nesting level and saving a line of code.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

The Kconfig currently controlling compilation of this code is:

init/Kconfig:config TREE_RCU_TRACE
init/Kconfig:   def_bool RCU_TRACE && ( TREE_RCU || PREEMPT_RCU )

...meaning that it currently is not being built as a module by anyone.

Lets remove the modular code that is essentially orphaned, so that
when reading the file there is no doubt it is builtin-only.

Since module_init translates to device_initcall in the non-modular
case, the init ordering remains unchanged with this commit.  We could
consider moving this to an earlier initcall if desired.

We don't replace module.h with init.h since the file already has that.
We also delete the moduleparam.h include that is left over from
commit 64db4cff (""Tree RCU": scalable
classic RCU implementation") since it is not needed here either.

We morph some tags like MODULE_AUTHOR into the comments at the top of
the file for documentation purposes.

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Currently, the rcu_node_class[], rcu_fqs_class[], and rcu_exp_class[]
arrays needlessly pollute the global namespace within tree.c.  This
commit therefore converts them to static local variables within
rcu_init_one().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Expedited grace periods can speed up boot, but are undesirable in
aggressive real-time systems.  This commit therefore introduces a
kernel parameter rcupdate.rcu_normal_after_boot that disables
expedited grace periods just before init is spawned.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit adds the invocation of rcu_end_inkernel_boot() just before
init is invoked.  This allows the CONFIG_RCU_EXPEDITE_BOOT Kconfig
option to do something useful and prepares for the upcoming
rcupdate.rcu_normal_after_boot kernel parameter.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Although expedited grace periods can be quite useful, and although their
OS jitter has been greatly reduced, they can still pose problems for
extreme real-time workloads.  This commit therefore adds a rcu_normal
kernel boot parameter (which can also be manipulated via sysfs)
to suppress expedited grace periods, that is, to treat requests for
expedited grace periods as if they were requests for normal grace periods.
If both rcu_expedited and rcu_normal are specified, rcu_normal wins.
This means that if you are relying on expedited grace periods to speed up
boot, you will want to specify rcu_expedited on the kernel command line,
and then specify rcu_normal via sysfs once boot completes.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit adds print statements that check the rcu_node structure to
find which ->expmask bits and which ->exp_tasks structures are blocking
the current expedited grace period.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>