linux-kernel - Re: [PATCH 2/4] block: fix ordering between checking BLK_MQ_S_STOPPED and adding requests to hctx->dispatch

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Message-ID: <786a8d94-884c-8a31-151d-fdc82e1a0a63@huaweicloud.com>
Date: Thu, 22 Aug 2024 11:54:44 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Muchun Song <songmuchun@...edance.com>, Ming Lei <ming.lei@...hat.com>
Cc: Jens Axboe <axboe@...nel.dk>,
 "open list:BLOCK LAYER" <linux-block@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>, muchun.song@...ux.dev,
 "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH 2/4] block: fix ordering between checking BLK_MQ_S_STOPPED
 and adding requests to hctx->dispatch

Hi,

在 2024/08/19 11:49, Muchun Song 写道:
> On Mon, Aug 19, 2024 at 10:28 AM Ming Lei <ming.lei@...hat.com> wrote:
>>
>> Hi Muchun,
>>
>> On Sun, Aug 11, 2024 at 06:19:19PM +0800, Muchun Song wrote:
>>> Supposing the following scenario with a virtio_blk driver.
>>>
>>> CPU0                                                                CPU1
>>>
>>> blk_mq_try_issue_directly()
>>>      __blk_mq_issue_directly()
>>>          q->mq_ops->queue_rq()
>>>              virtio_queue_rq()
>>>                  blk_mq_stop_hw_queue()
>>>                                                                      virtblk_done()
>>>      blk_mq_request_bypass_insert()                                      blk_mq_start_stopped_hw_queues()
>>>          /* Add IO request to dispatch list */   1) store                    blk_mq_start_stopped_hw_queue()
>>>                                                                                  clear_bit(BLK_MQ_S_STOPPED)                 3) store
>>>      blk_mq_run_hw_queue()                                                       blk_mq_run_hw_queue()
>>>          if (!blk_mq_hctx_has_pending())                                             if (!blk_mq_hctx_has_pending())         4) load
>>>              return                                                                      return
>>>          blk_mq_sched_dispatch_requests()                                            blk_mq_sched_dispatch_requests()
>>>              if (blk_mq_hctx_stopped())          2) load                                 if (blk_mq_hctx_stopped())
>>>                  return                                                                      return
>>>              __blk_mq_sched_dispatch_requests()                                          __blk_mq_sched_dispatch_requests()
>>>
>>> The full memory barrier should be inserted between 1) and 2), as well as between
>>> 3) and 4) to make sure that either CPU0 sees BLK_MQ_S_STOPPED is cleared or CPU1
>>> sees dispatch list or setting of bitmap of software queue. Otherwise, either CPU
>>> will not re-run the hardware queue causing starvation.
>>
>> Yeah, it is one kind of race which is triggered when adding request into
>> ->dispatch list after returning STS_RESOURCE. We were troubled by lots of
>> such kind of race.
> 
> Yes. I saw the similar fix for BLK_MQ_S_SCHED_RESTART.
> 
>>
>> stopping queue is used in very less drivers, and its only purpose should
>> be for throttling hw queue in case that low level queue is busy. There seems
>> more uses of blk_mq_stop_hw_queues(), but most of them should be replaced
>> with blk_mq_quiesce_queue().
>>
>> IMO, fixing this kind of issue via memory barrier is too tricky to
>> maintain cause WRITE/READ dependency is very hard to follow. I'd suggest to
>> make memory barrier solution as the last resort, and we can try to figure
>> out other easier & more reliable way first.
> 
> I do agree it is hard to maintain the dependencies in the future. We should
> propose an easy-maintainable solution. But I thought it is a long-term issue
> throughout different stable linux distros. Adding a mb is the easy way to fix
> the problem (the code footprint is really small), so it will be very
> easy for others
> to backport those bug fixes to different stable linux distros. Therefore, mb
> should be an interim solution. Then, we could improve it based on the solution
> you've proposed below. What do you think?

I'll agree with Ming, let's figure out a better fix first. Easy to 
backport to stables is not first consideration.
> 
> Thanks,
> Muchun.
> 
>>
>> One idea I thought of is to call blk_mq_request_bypass_insert()(or rename
>> & export it) before calling blk_mq_stop_hw_queue() in driver, then
>> return new status code STS_STOP_DISPATCH for notifying blk-mq to stop
>> dispatch simply.

New status code look good to me, however, I wonder can we just remove
the problematic blk_mq_stop_hw_queue(), and replace it by handling the
new status from block layer?

- Passing the new status to blk_mq_run_dispatch_ops, and quiesce with
the new status, if no request is inflight, unquiesce immediately;
- unquiesce is any IO is done afterwards;

Thanks,
Kuai

>>
>>
>> thanks,
>> Ming
>>
> 
> .
>