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From: "Chanho Min" <chanho.min@lge.com>
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Subject: RE: [PATCH] mm/backing-dev.c: fix crash when USB/SCSI device is detached
Date: Tue, 3 Jan 2012 12:23:44 +0900
Message-ID: <002e01ccc9c7$1928c940$4b7a5bc0$@min@lge.com>
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Sender: owner-linux-mm@kvack.org
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To: 'Wu Fengguang' <fengguang.wu@intel.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, 'Jens Axboe' <axboe@kernel.dk>, 'Andrew Morton' <akpm@linux-foundation.org>

>On Mon, Jan 02, 2012 at 06:38:21PM +0900, =
=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=C8=A3 wrote:
>> from Chanho Min <chanho.min@lge.com>
>>
>> System may crash in backing-dev.c when removal SCSI device is =
detached.
>> bdi task is killed by bdi_unregister()/'khubd', but task's point =
remains.
>> Shortly afterward, If 'wb->wakeup_timer' is expired before
>> del_timer()/bdi_forker_thread,
>> wakeup_timer_fn() may wake up the dead thread which cause the crash.
>> 'bdi->wb.task' should be NULL as this patch.
>
>Is it some race condition between del_timer() and del_timer_sync()?
>
>bdi_unregister() calls
>
>        del_timer_sync
>        bdi_wb_shutdown
>            kthread_stop
>
>in turn, and del_timer_sync() should guarantee wakeup_timer_fn() is
>no longer called to access the stopped task.
>

It is not race condition. This happens when USB is removed during =
write-access.
bdi_wakeup_thread_delayed is called after kthread_stop, and timer is =
activated again.

	bdi_unregister
		kthread_stop
	bdi_wakeup_thread_delayed (sys_write mostly calls this)
	timer fires

Anyway,Is this safeguard to prevent from waking up killed thread?

Thanks,
Chanho

>Thanks,
>Fengguang
>
>
>> Signed-off-by: Chanho Min <chanho.min@lge.com>
>> ---
>>  mm/backing-dev.c |    1 +
>>  1 files changed, 1 insertions(+), 0 deletions(-)
>>
>> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
>> index 71034f4..4378a5e 100644
>> --- a/mm/backing-dev.c
>> +++ b/mm/backing-dev.c
>> @@ -607,6 +607,7 @@ static void bdi_wb_shutdown(struct =
backing_dev_info
>> *bdi)
>>         if (bdi->wb.task) {
>>                 thaw_process(bdi->wb.task);
>>                 kthread_stop(bdi->wb.task);
>> +               bdi->wb.task =3D NULL;
>>         }
>>  }
>>
>> --
>> 1.7.0.4

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