linux-kernel - Re: [PATCH 3/2] oom: clear TIF_MEMDIE after oom_reaper managed to unmap the address space

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Message-ID: <20160128223351.GA14803@dhcp22.suse.cz>
Date:	Thu, 28 Jan 2016 23:33:51 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Johannes Weiner <hannes@...xchg.org>, Mel Gorman <mgorman@...e.de>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	David Rientjes <rientjes@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrea Argangeli <andrea@...nel.org>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/2] oom: clear TIF_MEMDIE after oom_reaper managed to
 unmap the address space

I have missed one important point. exit_oom_victim has to do
test_and_clear to make sure we do not race now with this patch. So we
need to fold the following into the patch
---
>From 296166b049ceb8b3199019a24aebab032998ebb0 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.com>
Date: Thu, 28 Jan 2016 23:27:26 +0100
Subject: [PATCH] 
 oom-clear-tif_memdie-after-oom_reaper-managed-to-unmap-the-address-space-fix

Now that exit_oom_victim might be called on a remote task from
__oom_reap_task we have to check and clear the flag atomically
otherwise we might race and underflow oom_victims or wake up
waiters too early.

Signed-off-by: Michal Hocko <mhocko@...e.com>
---
 mm/oom_kill.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 7209e517adf2..8f5488345c42 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -603,7 +603,8 @@ void mark_oom_victim(struct task_struct *tsk)
  */
 void exit_oom_victim(struct task_struct *tsk)
 {
-	clear_tsk_thread_flag(tsk, TIF_MEMDIE);
+	if (!test_and_clear_tsk_thread_flag(tsk, TIF_MEMDIE))
+		return;
 
 	if (!atomic_dec_return(&oom_victims))
 		wake_up_all(&oom_victims_wait);
-- 
2.7.0.rc3

-- 
Michal Hocko
SUSE Labs