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

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Date:	Fri, 20 Jan 2012 20:18:31 +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 03:15:32PM +0900, Namjae Jeon wrote:
>> 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 ?
>
> wakeup_timer_fn() holds the wb_lock across the check for bdi->dev !=
> NULL and the call to trace_writeback_wake_forker_thread(), so no.

The following case is not possible with racing of __mark_inode_dirty  ?
--------------------------------------------------------------------------------------------------
spin_lock
bdi->dev = NULL;
spin_unlock
                              spin_lock
                               ......
                               ......
                               trace_writeback_wake_forker_thread(bdi);
                               spin_unlock
-----------------------------------------------------------------------------
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