From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752821AbbCZMxy (ORCPT ); Thu, 26 Mar 2015 08:53:54 -0400 Received: from cantor2.suse.de ([195.135.220.15]:43910 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752631AbbCZMxv (ORCPT ); Thu, 26 Mar 2015 08:53:51 -0400 Date: Thu, 26 Mar 2015 13:53:48 +0100 From: Michal Hocko To: Johannes Weiner Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Linus Torvalds , Andrew Morton , Tetsuo Handa , Huang Ying , Andrea Arcangeli , Dave Chinner , "Theodore Ts'o" Subject: Re: [patch 04/12] mm: oom_kill: remove unnecessary locking in exit_oom_victim() Message-ID: <20150326125348.GF15257@dhcp22.suse.cz> References: <1427264236-17249-1-git-send-email-hannes@cmpxchg.org> <1427264236-17249-5-git-send-email-hannes@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1427264236-17249-5-git-send-email-hannes@cmpxchg.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 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 > --- > 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