[PATCH] sched/completion: completion_done() should serialize with complete()

From: Oleg Nesterov
Date: Thu Feb 12 2015 - 15:01:24 EST

Commit de30ec47302c "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())

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@xxxxxxxxxxxxxxxxxx>
Reported-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>

--- x/kernel/sched/completion.c
+++ x/kernel/sched/completion.c
@@ -274,7 +274,7 @@ bool try_wait_for_completion(struct comp
* 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 +297,11 @@ EXPORT_SYMBOL(try_wait_for_completion);
bool completion_done(struct completion *x)
- return !!ACCESS_ONCE(x->done);
+ if (!READ_ONCE(x->done))
+ return false;
+ smp_rmb();
+ spin_unlock_wait(&x->wait.lock);
+ return true;

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