linux-kernel - Re: [PATCHSET] kthread_worker: reimplement flush_kthread

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Message-ID: <20120917194016.GI18677@google.com>
Date:	Mon, 17 Sep 2012 12:40:16 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Colin Cross <ccross@...gle.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org,
	Andy Walls <awalls@...metrocast.net>,
	ivtv-devel@...vdriver.org, linux-media@...r.kernel.org,
	Grant Likely <grant.likely@...retlab.ca>,
	spi-devel-general@...ts.sourceforge.net,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	stable@...r.kernel.org
Subject: Re: [PATCHSET] kthread_worker: reimplement flush_kthread_work() to
 allow freeing during execution

On Fri, Sep 14, 2012 at 03:50:40PM -0700, Colin Cross wrote:
> This patch set fixes a reproducible crash I'm seeing on a 3.4.10
> kernel.  flush_kthread_worker (which is different from
> flush_kthread_work) is initializing a kthread_work and a completion on
> the stack, then queuing it and calling wait_for_completion.  Once the
> completion is signaled, flush_kthread_worker exits and the stack
> region used by the kthread_work may be immediately reused by another
> object on the stack, but kthread_worker_fn continues accessing its
> work pointer:
>                 work->func(work);         <- calls complete,
> effectively frees work
>                 smp_wmb();      /* wmb worker-b0 paired with flush-b1 */
>                 work->done_seq = work->queue_seq;   <- overwrites a
> new stack object
>                 smp_mb();       /* mb worker-b1 paired with flush-b0 */
>                 if (atomic_read(&work->flushing))
>                         wake_up_all(&work->done);  <- or crashes here
> 
> These patches fix the problem by not accessing work after work->func
> is called, and should be backported to stable.  They apply cleanly to
> 3.4.10.  Upstream commits are 9a2e03d8ed518a61154f18d83d6466628e519f94
> and 46f3d976213452350f9d10b0c2780c2681f7075b.

Yeah, you're right.  I wonder why this didn't come up before.  Greg,
can you please pick up these two commits?

Thanks.

-- 
tejun
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