linux-kernel - Re: [PATCHv2] backing-dev: fix wakeup timer races with bdi

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Date:	Fri, 20 Jan 2012 10:54:11 +0530
From:	Rabin Vincent <rabin@....in>
To:	Namjae Jeon <linkinjeon@...il.com>
Cc:	fengguang.wu@...el.com, axboe@...nel.dk,
	linux-kernel@...r.kernel.org, chanho0207@...il.com
Subject: Re: [PATCHv2] backing-dev: fix wakeup timer races with bdi_unregister()

On Fri, Jan 20, 2012 at 05:16, Namjae Jeon <linkinjeon@...il.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.
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