Re: [PATCH v2 1/3] block: fix missing dispatching request when queue is started or unquiesced

Muchun Song
2024-09-11T02:44:40Z

> On Sep 10, 2024, at 21:17, Jens Axboe <axboe@kernel.dk> wrote:
>
> On 9/3/24 2:16 AM, Muchun Song wrote:
>> Supposing the following scenario with a virtio_blk driver.
>>
>> CPU0 CPU1 CPU2
>>
>> blk_mq_try_issue_directly()
>> __blk_mq_issue_directly()
>> q->mq_ops->queue_rq()
>> virtio_queue_rq()
>> blk_mq_stop_hw_queue()
>> blk_mq_try_issue_directly() virtblk_done()
>> if (blk_mq_hctx_stopped())
>> blk_mq_request_bypass_insert() blk_mq_start_stopped_hw_queue()
>> blk_mq_run_hw_queue() blk_mq_run_hw_queue()
>> blk_mq_insert_request()
>> return // Who is responsible for dispatching this IO request?
>>
>> After CPU0 has marked the queue as stopped, CPU1 will see the queue is stopped.
>> But before CPU1 puts the request on the dispatch list, CPU2 receives the interrupt
>> of completion of request, so it will run the hardware queue and marks the queue
>> as non-stopped. Meanwhile, CPU1 also runs the same hardware queue. After both CPU1
>> and CPU2 complete blk_mq_run_hw_queue(), CPU1 just puts the request to the same
>> hardware queue and returns. It misses dispatching a request. Fix it by running
>> the hardware queue explicitly. And blk_mq_request_issue_directly() should handle
>> a similar situation. Fix it as well.
>
> Patch looks fine, but this commit message is waaaaay too wide. Please
> limit it to 72-74 chars. The above ordering is diagram is going to
> otherwise be unreadable in a git log viewing in a terminal.

Thanks for your reply. I'll adjust those lines to make the digram more
readable.

Muchun,
Thanks.

>
> --
> Jens Axboe