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sched: fix the theoretical signal_wake_up() vs schedule() race

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This is only theoretical, but after try_to_wake_up(p) was changed
to check p->state under p->pi_lock the code like

	__set_current_state(TASK_INTERRUPTIBLE);
	schedule();

can miss a signal. This is the special case of wait-for-condition,
it relies on try_to_wake_up/schedule interaction and thus it does
not need mb() between __set_current_state() and if(signal_pending).

However, this __set_current_state() can move into the critical
section protected by rq->lock, now that try_to_wake_up() takes
another lock we need to ensure that it can't be reordered with
"if (signal_pending(current))" check inside that section.

The patch is actually one-liner, it simply adds smp_wmb() before
spin_lock_irq(rq->lock). This is what try_to_wake_up() already
does by the same reason.

We turn this wmb() into the new helper, smp_mb__before_spinlock(),
for better documentation and to allow the architectures to change
the default implementation.

While at it, kill smp_mb__after_lock(), it has no callers.

Perhaps we can also add smp_mb__before/after_spinunlock() for
prepare_to_wait().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Oleg Nesterov 2013-08-12 18:14:00 +02:00 • committed by Linus Torvalds
parent 584d88b2cd
commit e0acd0a68e
3 changed files with 24 additions and 8 deletions
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14
include/linux/spinlock.h
View file

@ -117,9 +117,17 @@ do { \
#endif /*arch_spin_is_contended*/
#endif
/* The lock does not imply full memory barrier. */
#ifndef ARCH_HAS_SMP_MB_AFTER_LOCK
static inline void smp_mb__after_lock(void) { smp_mb(); }
/*
* Despite its name it doesn't necessarily has to be a full barrier.
* It should only guarantee that a STORE before the critical section
* can not be reordered with a LOAD inside this section.
* spin_lock() is the one-way barrier, this LOAD can not escape out
* of the region. So the default implementation simply ensures that
* a STORE can not move into the critical section, smp_wmb() should
* serialize it with another STORE done by spin_lock().
*/
#ifndef smp_mb__before_spinlock
#define smp_mb__before_spinlock() smp_wmb()
#endif
/**