Messages in this thread Patch in this message | | | Date | Thu, 12 Feb 2015 20:59:13 +0100 | From | Oleg Nesterov <> | Subject | [PATCH] sched/completion: completion_done() should serialize with complete() |
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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()) 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> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
--- 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; } EXPORT_SYMBOL(completion_done);
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