linux-kernel - [PATCH] oom_reaper: don't call mmput

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Date:	Fri, 27 May 2016 10:00:48 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	David Rientjes <rientjes@...gle.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Johannes Weiner <hannes@...xchg.org>,
	Oleg Nesterov <oleg@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] oom_reaper: don't call mmput_async() on uninitialized mm

The change to the oom_reaper to hold a mutex inside __oom_reap_task()
accidentally started calling mmput_async() on the local
mm before that variable got initialized, as reported by gcc
in linux-next:

mm/oom_kill.c: In function '__oom_reap_task':
mm/oom_kill.c:537:2: error: 'mm' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This rearranges the code slightly back to the state before patch
but leaves the lock in place. The error handling in the function
still looks a bit confusing and could probably be improved
but I could not come up with a solution that made me happy
for now.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
Fixes: mmotm ("oom_reaper: close race with exiting task")
---
 mm/oom_kill.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 1685890d424e..255cb5f48019 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -447,7 +447,7 @@ static bool __oom_reap_task(struct task_struct *tsk)
 	struct task_struct *p;
 	struct zap_details details = {.check_swap_entries = true,
 				      .ignore_dirty = true};
-	bool ret = true;
+	bool ret;
 
 	/*
 	 * We have to make sure to not race with the victim exit path
@@ -472,13 +472,16 @@ static bool __oom_reap_task(struct task_struct *tsk)
 	 * is no mm.
 	 */
 	p = find_lock_task_mm(tsk);
-	if (!p)
-		goto unlock_oom;
+	if (!p) {
+		mutex_unlock(&oom_lock);
+		return true;
+	}
 
 	mm = p->mm;
 	if (!atomic_inc_not_zero(&mm->mm_users)) {
 		task_unlock(p);
-		goto unlock_oom;
+		mutex_unlock(&oom_lock);
+		return true;
 	}
 
 	task_unlock(p);
@@ -527,6 +530,7 @@ static bool __oom_reap_task(struct task_struct *tsk)
 	 * to release its memory.
 	 */
 	set_bit(MMF_OOM_REAPED, &mm->flags);
+	ret = true;
 unlock_oom:
 	mutex_unlock(&oom_lock);
 	/*
-- 
2.7.0