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From: Rabin Vincent <rabin@rab.in>
To: Namjae Jeon <linkinjeon@gmail.com>
Cc: fengguang.wu@intel.com, axboe@kernel.dk,
	linux-kernel@vger.kernel.org, chanho0207@gmail.com
Subject: Re: [PATCHv2] backing-dev: fix wakeup timer races with bdi_unregister()
Date: Fri, 20 Jan 2012 10:54:11 +0530	[thread overview]
Message-ID: <CAH+eYFBHTXVxdepno4xE-AxyYKqtNQHUvCY55z9_otN+25Y8Hg@mail.gmail.com> (raw)
In-Reply-To: <CAKYAXd8poi6Rs7wyvZHre6qx-_i7eq0SBQAJNKNTYTLC5RQKjA@mail.gmail.com>

On Fri, Jan 20, 2012 at 05:16, Namjae Jeon <linkinjeon@gmail.com> wrote:
>>                bdi_debug_unregister(bdi);
>> -               device_unregister(bdi->dev);
>> +
>> +               spin_lock_bh(&bdi->wb_lock);
>>                bdi->dev = NULL;
>> +               spin_unlock_bh(&bdi->wb_lock);
> Hi.
> Would you explain me why you add spinlock in here ?

wakeup_timer_fn() does the following, where the
trace_writeback_wake_forker_thread() also accesses bdi->dev.
It does this under the wb_lock:

	} else if (bdi->dev) {
		/*
		 * When bdi tasks are inactive for long time, they are killed.
		 * In this case we have to wake-up the forker thread which
		 * should create and run the bdi thread.
		 */
		trace_writeback_wake_forker_thread(bdi);

If we don't have the lock above, the bdi->dev could potentially be
cleared after the check but before the tracepoint is hit, leading to a
NULL pointer dereference.

  reply	other threads:[~2012-01-20  5:24 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-01-05  8:49 [PATCH] mm/backing-dev.c: fix crash when USB/SCSI device is detached Chanho Min
2012-01-15 10:28 ` Rabin Vincent
2012-01-15 12:58   ` Wu Fengguang
2012-01-15 15:41     ` Rabin Vincent
2012-01-16  2:53       ` Wu Fengguang
2012-01-16  5:28         ` Chanho Min
2012-01-16  5:50           ` Wu Fengguang
2012-01-16  5:53             ` Wu Fengguang
2012-01-16  6:34               ` Chanho Min
2012-01-18 19:43                 ` Rabin Vincent
2012-01-19 16:50         ` [PATCHv2] backing-dev: fix wakeup timer races with bdi_unregister() Rabin Vincent
2012-01-19 23:46           ` Namjae Jeon
2012-01-20  5:24             ` Rabin Vincent [this message]
2012-01-20  6:15               ` Namjae Jeon
2012-01-20 10:03                 ` Rabin Vincent
2012-01-20 11:18                   ` Namjae Jeon
2012-01-20 12:08                     ` Rabin Vincent
2012-01-20 15:04                       ` Namjae Jeon
2012-01-31 13:24           ` Wu Fengguang

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