From: Namjae Jeon <linkinjeon@gmail.com>
To: Rabin Vincent <rabin@rab.in>
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 20:18:31 +0900 [thread overview]
Message-ID: <CAKYAXd8UuC0pAX7G7aG2q_3U_bPG8Bs_GpjxbfiX=9=Q+RW1Rg@mail.gmail.com> (raw)
In-Reply-To: <20120120100309.GA20640@debian>
2012/1/20 Rabin Vincent <rabin@rab.in>:
> On Fri, Jan 20, 2012 at 03:15:32PM +0900, Namjae Jeon wrote:
>> 2012/1/20 Rabin Vincent <rabin@rab.in>:
>> > 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.
>> 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
-----------------------------------------------------------------------------
next prev parent reply other threads:[~2012-01-20 11:18 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
2012-01-20 6:15 ` Namjae Jeon
2012-01-20 10:03 ` Rabin Vincent
2012-01-20 11:18 ` Namjae Jeon [this message]
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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