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

Jens Axboe
2024-09-10T13:17:33Z

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.

--
Jens Axboe