Commit fb0527bd authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar

locking/mutexes: Introduce cancelable MCS lock for adaptive spinning

Since we want a task waiting for a mutex_lock() to go to sleep and
reschedule on need_resched() we must be able to abort the
mcs_spin_lock() around the adaptive spin.

Therefore implement a cancelable mcs lock.
Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
Cc: chegu_vinod@hp.com
Cc: paulmck@linux.vnet.ibm.com
Cc: Waiman.Long@hp.com
Cc: torvalds@linux-foundation.org
Cc: tglx@linutronix.de
Cc: riel@redhat.com
Cc: akpm@linux-foundation.org
Cc: davidlohr@hp.com
Cc: hpa@zytor.com
Cc: andi@firstfloor.org
Cc: aswin@hp.com
Cc: scott.norton@hp.com
Cc: Jason Low <jason.low2@hp.com>
Link: http://lkml.kernel.org/n/tip-62hcl5wxydmjzd182zhvk89m@git.kernel.orgSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 1d8fe7dc
......@@ -46,7 +46,7 @@
* - detects multi-task circular deadlocks and prints out all affected
* locks and tasks (and only those tasks)
*/
struct mcs_spinlock;
struct optimistic_spin_queue;
struct mutex {
/* 1: unlocked, 0: locked, negative: locked, possible waiters */
atomic_t count;
......@@ -56,7 +56,7 @@ struct mutex {
struct task_struct *owner;
#endif
#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
struct mcs_spinlock *mcs_lock; /* Spinner MCS lock */
struct optimistic_spin_queue *osq; /* Spinner MCS lock */
#endif
#ifdef CONFIG_DEBUG_MUTEXES
const char *name;
......
obj-y += mutex.o semaphore.o rwsem.o lglock.o
obj-y += mutex.o semaphore.o rwsem.o lglock.o mcs_spinlock.o
ifdef CONFIG_FUNCTION_TRACER
CFLAGS_REMOVE_lockdep.o = -pg
......
#include <linux/percpu.h>
#include <linux/mutex.h>
#include <linux/sched.h>
#include "mcs_spinlock.h"
#ifdef CONFIG_SMP
/*
* An MCS like lock especially tailored for optimistic spinning for sleeping
* lock implementations (mutex, rwsem, etc).
*
* Using a single mcs node per CPU is safe because sleeping locks should not be
* called from interrupt context and we have preemption disabled while
* spinning.
*/
static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_queue, osq_node);
/*
* Get a stable @node->next pointer, either for unlock() or unqueue() purposes.
* Can return NULL in case we were the last queued and we updated @lock instead.
*/
static inline struct optimistic_spin_queue *
osq_wait_next(struct optimistic_spin_queue **lock,
struct optimistic_spin_queue *node,
struct optimistic_spin_queue *prev)
{
struct optimistic_spin_queue *next = NULL;
for (;;) {
if (*lock == node && cmpxchg(lock, node, prev) == node) {
/*
* We were the last queued, we moved @lock back. @prev
* will now observe @lock and will complete its
* unlock()/unqueue().
*/
break;
}
/*
* We must xchg() the @node->next value, because if we were to
* leave it in, a concurrent unlock()/unqueue() from
* @node->next might complete Step-A and think its @prev is
* still valid.
*
* If the concurrent unlock()/unqueue() wins the race, we'll
* wait for either @lock to point to us, through its Step-B, or
* wait for a new @node->next from its Step-C.
*/
if (node->next) {
next = xchg(&node->next, NULL);
if (next)
break;
}
arch_mutex_cpu_relax();
}
return next;
}
bool osq_lock(struct optimistic_spin_queue **lock)
{
struct optimistic_spin_queue *node = this_cpu_ptr(&osq_node);
struct optimistic_spin_queue *prev, *next;
node->locked = 0;
node->next = NULL;
node->prev = prev = xchg(lock, node);
if (likely(prev == NULL))
return true;
ACCESS_ONCE(prev->next) = node;
/*
* Normally @prev is untouchable after the above store; because at that
* moment unlock can proceed and wipe the node element from stack.
*
* However, since our nodes are static per-cpu storage, we're
* guaranteed their existence -- this allows us to apply
* cmpxchg in an attempt to undo our queueing.
*/
while (!smp_load_acquire(&node->locked)) {
/*
* If we need to reschedule bail... so we can block.
*/
if (need_resched())
goto unqueue;
arch_mutex_cpu_relax();
}
return true;
unqueue:
/*
* Step - A -- stabilize @prev
*
* Undo our @prev->next assignment; this will make @prev's
* unlock()/unqueue() wait for a next pointer since @lock points to us
* (or later).
*/
for (;;) {
if (prev->next == node &&
cmpxchg(&prev->next, node, NULL) == node)
break;
/*
* We can only fail the cmpxchg() racing against an unlock(),
* in which case we should observe @node->locked becomming
* true.
*/
if (smp_load_acquire(&node->locked))
return true;
arch_mutex_cpu_relax();
/*
* Or we race against a concurrent unqueue()'s step-B, in which
* case its step-C will write us a new @node->prev pointer.
*/
prev = ACCESS_ONCE(node->prev);
}
/*
* Step - B -- stabilize @next
*
* Similar to unlock(), wait for @node->next or move @lock from @node
* back to @prev.
*/
next = osq_wait_next(lock, node, prev);
if (!next)
return false;
/*
* Step - C -- unlink
*
* @prev is stable because its still waiting for a new @prev->next
* pointer, @next is stable because our @node->next pointer is NULL and
* it will wait in Step-A.
*/
ACCESS_ONCE(next->prev) = prev;
ACCESS_ONCE(prev->next) = next;
return false;
}
void osq_unlock(struct optimistic_spin_queue **lock)
{
struct optimistic_spin_queue *node = this_cpu_ptr(&osq_node);
struct optimistic_spin_queue *next;
/*
* Fast path for the uncontended case.
*/
if (likely(cmpxchg(lock, node, NULL) == node))
return;
/*
* Second most likely case.
*/
next = xchg(&node->next, NULL);
if (next) {
ACCESS_ONCE(next->locked) = 1;
return;
}
next = osq_wait_next(lock, node, NULL);
if (next)
ACCESS_ONCE(next->locked) = 1;
}
#endif
......@@ -111,4 +111,19 @@ void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
arch_mcs_spin_unlock_contended(&next->locked);
}
/*
* Cancellable version of the MCS lock above.
*
* Intended for adaptive spinning of sleeping locks:
* mutex_lock()/rwsem_down_{read,write}() etc.
*/
struct optimistic_spin_queue {
struct optimistic_spin_queue *next, *prev;
int locked; /* 1 if lock acquired */
};
extern bool osq_lock(struct optimistic_spin_queue **lock);
extern void osq_unlock(struct optimistic_spin_queue **lock);
#endif /* __LINUX_MCS_SPINLOCK_H */
......@@ -53,7 +53,7 @@ __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
INIT_LIST_HEAD(&lock->wait_list);
mutex_clear_owner(lock);
#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
lock->mcs_lock = NULL;
lock->osq = NULL;
#endif
debug_mutex_init(lock, name, key);
......@@ -403,7 +403,9 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
if (!mutex_can_spin_on_owner(lock))
goto slowpath;
mcs_spin_lock(&lock->mcs_lock, &node);
if (!osq_lock(&lock->osq))
goto slowpath;
for (;;) {
struct task_struct *owner;
......@@ -442,7 +444,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
}
mutex_set_owner(lock);
mcs_spin_unlock(&lock->mcs_lock, &node);
osq_unlock(&lock->osq);
preempt_enable();
return 0;
}
......@@ -464,7 +466,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
*/
arch_mutex_cpu_relax();
}
mcs_spin_unlock(&lock->mcs_lock, &node);
osq_unlock(&lock->osq);
slowpath:
#endif
spin_lock_mutex(&lock->wait_lock, flags);
......
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