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

[Muchun Song <muchun.song@linux.dev>]

> On Aug 22, 2024, at 11:54, Yu Kuai <yukuai1@huaweicloud.com> wrote:
> 
> Hi,
> 
> 在 2024/08/19 11:49, Muchun Song 写道:
>> On Mon, Aug 19, 2024 at 10:28 AM Ming Lei <ming.lei@redhat.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.

Hi Kuai,

All right. I usually focus on MM, it seems there is a gap between MM and BLock.
Anyway, let's figure out if there is any good solution.

>> 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

I didn't fully understand your suggestion. Let me ask some questions.
blk_mq_stop_hw_queue() is usually called in blk_mq_ops->queue_rq path,
it'll be easy for this case to pass the new status to blk_mq_run_dispatch_ops.
Should we remove blk_mq_stop_hw_queues() as well? How to pass the new
status to blk_mq_run_dispatch_ops in this case?

> the new status, if no request is inflight, unquiesce immediately;

Actually, I didn't understand how to avoid the above race. May you elaborate
the scenario?

Muhcun,
Thanks.

> - unquiesce is any IO is done afterwards;