linux-kernel - Re: [PATCH v4 6/7] blk-mq: fix freeze queue race

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Message-ID: <CAC5umygUfXMjaO=+nA1J5DqJ=tGfRfaPO4O5SCA=i9-NHfS1PA@mail.gmail.com>
Date:	Wed, 30 Sep 2015 19:32:11 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jens Axboe <axboe@...nel.dk>, LKML <linux-kernel@...r.kernel.org>,
	Ming Lei <tom.leiming@...il.com>,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v4 6/7] blk-mq: fix freeze queue race

2015-09-30 6:50 GMT+09:00 Tejun Heo <tj@...nel.org>:
> Hello,
>
> On Tue, Sep 29, 2015 at 11:03:46AM -0400, Tejun Heo wrote:
>> On Tue, Sep 29, 2015 at 09:01:31AM -0600, Jens Axboe wrote:
>> > I think that would be very useful, it seems sort of half-assed if the caller
>> > side has to provide serialization for that.
>>
>> Yeah, the thing is init/exit are usually caller synchronized but
>> percpu_rwsem's kill/reinit are more of mode-switching operations which
>> can be performed concurrently during operation so I think the right
>> thing to do here is making it synchronize itself.  Will spin a patch.
>
> Patchset posted
>
>  http://lkml.kernel.org/g/1443563240-29306-1-git-send-email-tj@kernel.org

Thanks.  So far it's working well without any problems.

Note that we still need a part of patch 6/7.
Quoted from this patch description:

        When unfreezing queue, there is a small window between decrementing
        q->mq_freeze_depth to zero and percpu_ref_reinit() call with
        q->mq_usage_counter.  If the other calls blk_mq_freeze_queue_start()
        in the window, q->mq_freeze_depth is increased from zero to one and
        percpu_ref_kill() is called with q->mq_usage_counter which is already
        killed.  percpu refcount should be re-initialized before killed again.

So we don't need to protect percpu_ref_switch_to_percpu() in
blk_mq_finish_init() anymore by your percpu_ref patchset, but we still
need to serialize blk_mq_freeze_queue_start() and blk_mq_unfreeze_queue().
(As you suggested earlier in this thread, q->mq_freeze_depth don't have
to be atomic_t anymore)
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