Commit bc956015 authored Feb 12, 2015 by Oleg Nesterov Committed by Ingo Molnar Feb 18, 2015

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



Commit de30ec47 "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: de30ec47 ("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>

parent 06b1f808

kernel/sched/completion.c

+17 −2

Original line number	Diff line number	Diff line
		@@ -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);