Messages in this thread | | | Date | Tue, 9 Oct 2012 17:17:29 +0200 | From | Oleg Nesterov <> | Subject | Re: [REPOST] RFC: sched: Prevent wakeup to enter critical section needlessly |
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On 10/09, Peter Zijlstra wrote: > > One thing you might need to consider is the memory ordering, will the > list_empty -- either careful or not -- observe the right list pointer, > or could it -- when racing with wait_event()/prepare_to_wait() -- > observe a stale value. Or.. is that all already covered in on the use > site.
I agree.
Without spin_lock(q->lock) (or some other barriers) wait_event-like code can miss an event.
wait_event:
prepare_to_wait(wq) // takes wq->lock
if (!CONDITION) schedule();
Now,
CONDITION = 1; wake_up(wq);
at least need the full mb() before lits_empty().
Oleg.
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