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

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Date:	Fri, 20 Jan 2012 15:15:32 +0900
From:	Namjae Jeon <linkinjeon@...il.com>
To:	Rabin Vincent <rabin@....in>
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()

2012/1/20 Rabin Vincent <rabin@....in>:
> 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.
Is there no possibility trace_writeback_wake_forker_thread is called
after spin_unlock of bdi->de= null ?
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