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

From: Akinobu Mita
Date: Wed Sep 30 2015 - 06:32:19 EST

2015-09-30 6:50 GMT+09:00 Tejun Heo <tj@xxxxxxxxxx>:
> 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

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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