Re: [patch 04/12] mm: oom_kill: remove unnecessary locking in exit_oom_victim()

Michal Hocko
2015-03-26T12:53:54Z

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