Messages in this thread Patch in this message | | | Date | Thu, 26 Mar 2015 13:53:48 +0100 | From | Michal Hocko <> | Subject | Re: [patch 04/12] mm: oom_kill: remove unnecessary locking in exit_oom_victim() |
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On Wed 25-03-15 02:17:08, Johannes Weiner wrote: > Disabling the OOM killer needs to exclude allocators from entering, > not existing victims from exiting.
The idea was that exit_oom_victim doesn't miss a waiter.
exit_oom_victim is doing atomic_dec_return(&oom_victims) && oom_killer_disabled)
so there is a full (implicit) memory barrier befor oom_killer_disabled check. The other part is trickier. oom_killer_disable does: oom_killer_disabled = true; up_write(&oom_sem);
wait_event(oom_victims_wait, !atomic_read(&oom_victims));
up_write doesn't guarantee a full memory barrier AFAICS in Documentation/memory-barriers.txt (although the generic and x86 implementations seem to implement it as a full barrier) but wait_event implies the full memory barrier (prepare_to_wait_event does spin lock&unlock) before checking the condition in the slow path. This should be sufficient and docummented...
/* * We do not need to hold oom_sem here because oom_killer_disable * guarantees that oom_killer_disabled chage is visible before * the waiter is put into sleep (prepare_to_wait_event) so * we cannot miss a wake up. */
in unmark_oom_victim()
> Right now the only waiter is suspend code, which achieves quiescence > by disabling the OOM killer. But later on we want to add waits that > hold the lock instead to stop new victims from showing up.
It is not entirely clear what you mean by this from the current context. exit_oom_victim is not called from any context which would be locked by any OOM internals so it should be safe to use the locking.
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
I have nothing against the change as it seems correct but it would be good to get a better clarification and also document the implicit memory barriers.
Acked-by: Michal Hocko <mhocko@suse.cz>
> --- > mm/oom_kill.c | 2 -- > 1 file changed, 2 deletions(-) > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c > index 4b9547be9170..88aa9ba40fa5 100644 > --- a/mm/oom_kill.c > +++ b/mm/oom_kill.c > @@ -437,14 +437,12 @@ void exit_oom_victim(void) > { > clear_thread_flag(TIF_MEMDIE); > > - down_read(&oom_sem); > /* > * There is no need to signal the lasst oom_victim if there > * is nobody who cares. > */ > if (!atomic_dec_return(&oom_victims) && oom_killer_disabled) > wake_up_all(&oom_victims_wait); > - up_read(&oom_sem); > } > > /** > -- > 2.3.3 >
-- Michal Hocko SUSE Labs
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