linux-kernel - Re: [PATCH] mm/backing-dev.c: fix crash when USB/SCSI device is detached

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Message-ID: <20120102095711.GA16570@localhost>
Date:	Mon, 2 Jan 2012 17:57:11 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	����ȣ <chanho.min@....com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	'Jens Axboe' <axboe@...nel.dk>,
	'Andrew Morton' <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm/backing-dev.c: fix crash when USB/SCSI device is
 detached

On Mon, Jan 02, 2012 at 06:38:21PM +0900, ����ȣ wrote:
> from Chanho Min <chanho.min@....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.

Thanks,
Fengguang


> Signed-off-by: Chanho Min <chanho.min@....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 = NULL;
>         }
>  }
> 
> -- 
> 1.7.0.4
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