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

[Muchun Song]

> On Aug 26, 2024, at 15:06, Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> 
> On Fri, Aug 23, 2024 at 7:28 PM Ming Lei <ming.lei@xxxxxxxxxx> wrote:
>> 
>> On Sun, Aug 11, 2024 at 06:19:21 PM +0800, Muchun Song wrote:
>>> Supposing the following scenario.
>>> 
>>> CPU0                                                                CPU1
>>> 
>>> blk_mq_request_issue_directly()                                     blk_mq_unquiesce_queue()
>>>    if (blk_queue_quiesced())                                           blk_queue_flag_clear(QUEUE_FLAG_QUIESCED)   3) store
>>>        blk_mq_insert_request()                                         blk_mq_run_hw_queues()
>>>            /*                                                              blk_mq_run_hw_queue()
>>>             * Add request to dispatch list or set bitmap of                    if (!blk_mq_hctx_has_pending())     4) load
>>>             * software queue.                  1) store                            return
>>>             */
>>>        blk_mq_run_hw_queue()
>>>            if (blk_queue_quiesced())           2) load
>>>                return
>>>            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 QUEUE_FLAG_QUIESCED 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.
>> 
>> Memory barrier shouldn't serve as bug fix for two slow code paths.
>> 
>> One simple fix is to add helper of blk_queue_quiesced_lock(), and
>> call the following check on CPU0:
>> 
>>        if (blk_queue_quiesced_lock())
>>         blk_mq_run_hw_queue();
> 
> This only fixes blk_mq_request_issue_directly(), I think anywhere that
> matching this
> pattern (inserting a request to dispatch list and then running the
> hardware queue)
> should be fixed. And I think there are many places which match this
> pattern (E.g.
> blk_mq_submit_bio()). The above graph should be adjusted to the following.
> 
> CPU0                                        CPU1
> 
> blk_mq_insert_request()         1) store    blk_mq_unquiesce_queue()
> blk_mq_run_hw_queue()
> blk_queue_flag_clear(QUEUE_FLAG_QUIESCED)       3) store
>    if (blk_queue_quiesced())   2) load         blk_mq_run_hw_queues()
>        return                                      blk_mq_run_hw_queue()
>    blk_mq_sched_dispatch_requests()                    if
> (!blk_mq_hctx_has_pending())     4) load
>                                                            return

Sorry. There is something wrong with my email client. Resend the graph.

CPU0                                        CPU1

blk_mq_insert_request()         1) store    blk_mq_unquiesce_queue()
blk_mq_run_hw_queue()                       blk_queue_flag_clear(QUEUE_FLAG_QUIESCED)       3) store
    if (blk_queue_quiesced())   2) load         blk_mq_run_hw_queues()
        return                                      blk_mq_run_hw_queue()
    blk_mq_sched_dispatch_requests()                    if (!blk_mq_hctx_has_pending())     4) load
                                                            return

> 
> So I think fixing blk_mq_run_hw_queue() could cover all of the situations.
> Maybe I thought wrongly. Please correct me.
> 
> Muchun,
> Thanks.