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@xxxxxxxxxxx> 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@xxxxxxx> > --- > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html