linux-kernel - Re: [PATCH 1/1] mm/backing-dev.c: Call del_timer_sync instead of del

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Message-Id: <20110901143333.51baf4ae.akpm@linux-foundation.org>
Date:	Thu, 1 Sep 2011 14:33:33 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Kautuk Consul <consul.kautuk@...il.com>
Cc:	Jens Axboe <jaxboe@...ionio.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Dave Chinner <dchinner@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/backing-dev.c: Call del_timer_sync instead of
 del_timer

On Thu,  1 Sep 2011 21:27:02 +0530
Kautuk Consul <consul.kautuk@...il.com> wrote:

> This is important for SMP scenario, to check whether the timer
> callback is executing on another CPU when we are deleting the
> timer.
> 

I don't see why?

> index d6edf8d..754b35a 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -385,7 +385,7 @@ static int bdi_forker_thread(void *ptr)
>  		 * dirty data on the default backing_dev_info
>  		 */
>  		if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list)) {
> -			del_timer(&me->wakeup_timer);
> +			del_timer_sync(&me->wakeup_timer);
>  			wb_do_writeback(me, 0);
>  		}

It isn't a use-after-free fix: bdi_unregister() safely shoots down any
running timer.

Please completely explain what you believe the problem is here.
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