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sched/completion: Serialize completion_done() with complete()

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Commit de30ec4730 "Remove unnecessary ->wait.lock serialization when
reading completion state" was not correct, without lock/unlock the code
like stop_machine_from_inactive_cpu()

	while (!completion_done())
		cpu_relax();

can return before complete() finishes its spin_unlock() which writes to
this memory. And spin_unlock_wait().

While at it, change try_wait_for_completion() to use READ_ONCE().

Reported-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reported-by: Davidlohr Bueso <dave@stgolabs.net>
Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
[ Added a comment with the barrier. ]
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nicholas Mc Guire <der.herr@hofr.at>
Cc: raghavendra.kt@linux.vnet.ibm.com
Cc: waiman.long@hp.com
Fixes: de30ec4730 ("sched/completion: Remove unnecessary ->wait.lock serialization when reading completion state")
Link: http://lkml.kernel.org/r/20150212195913.GA30430@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Oleg Nesterov 2015-02-12 20:59:13 +01:00 committed by Ingo Molnar
parent 06b1f8083d
commit bc9560155f
1 changed files with 17 additions and 2 deletions
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19
kernel/sched/completion.c
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@ -274,7 +274,7 @@ bool try_wait_for_completion(struct completion *x)
* first without taking the lock so we can
* return early in the blocking case.
*/
if (!ACCESS_ONCE(x->done))
if (!READ_ONCE(x->done))
return 0;
spin_lock_irqsave(&x->wait.lock, flags);
@ -297,6 +297,21 @@ EXPORT_SYMBOL(try_wait_for_completion);
*/
bool completion_done(struct completion *x)
{
return !!ACCESS_ONCE(x->done);
if (!READ_ONCE(x->done))
return false;
/*
* If ->done, we need to wait for complete() to release ->wait.lock
* otherwise we can end up freeing the completion before complete()
* is done referencing it.
*
* The RMB pairs with complete()'s RELEASE of ->wait.lock and orders
* the loads of ->done and ->wait.lock such that we cannot observe
* the lock before complete() acquires it while observing the ->done
* after it's acquired the lock.
*/
smp_rmb();
spin_unlock_wait(&x->wait.lock);
return true;
}
EXPORT_SYMBOL(completion_done);