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

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Date:	Fri, 27 May 2016 12:55:34 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michal Hocko <mhocko@...nel.org>
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: Re: [PATCH] oom_reaper: don't call mmput_async() on uninitialized
 mm

On Fri, 27 May 2016 10:10:59 +0200 Michal Hocko <mhocko@...nel.org> wrote:

> On Fri 27-05-16 10:00:48, Arnd Bergmann wrote:
> > 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")
> 
> Thanks for catching that Arnd?
> 
> Acked-by: Michal Hocko <mhocko@...e.com>

I think I preferred my version - all those unwinding return statements
can cause problems..

--- a/mm/oom_kill.c~oom_reaper-close-race-with-exiting-task-fix
+++ a/mm/oom_kill.c
@@ -443,7 +443,7 @@ static bool __oom_reap_task(struct task_
 {
 	struct mmu_gather tlb;
 	struct vm_area_struct *vma;
-	struct mm_struct *mm;
+	struct mm_struct *mm = NULL;
 	struct task_struct *p;
 	struct zap_details details = {.check_swap_entries = true,
 				      .ignore_dirty = true};
@@ -534,7 +534,8 @@ unlock_oom:
 	 * different context because we shouldn't risk we get stuck there and
 	 * put the oom_reaper out of the way.
 	 */
-	mmput_async(mm);
+	if (mm)
+		mmput_async(mm);
 	return ret;
 }
 
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