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From: Rusty Russell <rusty@rustcorp.com.au>
To: Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH -stable] kthreads: fix kthread_create() vs kthread_stop() race
Date: Wed, 26 Aug 2009 00:06:06 +0930
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Cc: "Greg Kroah-Hartman" <gregkh@suse.de>,
       Robert Peterson <rpeterso@redhat.com>, stable@kernel.org,
       linux-kernel@vger.kernel.org
References: <20090824104529.GA6899@redhat.com>
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On Mon, 24 Aug 2009 08:15:29 pm Oleg Nesterov wrote:
> The bug should be "accidently" fixed by recent changes in 2.6.31,
> all kernels <= 2.6.30 need the fix. The problem was never noticed before,
> it was found because it causes mysterious failures with GFS mount/umount.
> 
> Credits to Robert Peterson. He blaimed kthread.c from the very beginning.
> But, despite my promise, I forgot to inspect the old implementation until
> he did a lot of testing and reminded me. This led to huge delay in fixing
> this bug.
> 
> kthread_stop() does put_task_struct(k) before it clears kthread_stop_info.k.
> This means another kthread_create() can re-use this task_struct, but the
> new kthread can still see kthread_should_stop() == T and exit even without
> calling threadfn().
> 
> Reported-by: Robert Peterson <rpeterso@redhat.com>
> Tested-by: Robert Peterson <rpeterso@redhat.com>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Acked-by: Rusty Russell <rusty@rustcorp.com.au>

Thanks, good catch!
Rusty.