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

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Message-ID: <0e4e1f5a-30fd-430b-99ec-8b1004d8e3fd@kernel.dk>
Date: Tue, 10 Sep 2024 07:17:30 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Muchun Song <songmuchun@...edance.com>, ming.lei@...hat.com,
 yukuai1@...weicloud.com
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 muchun.song@...ux.dev, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/3] block: fix missing dispatching request when queue
 is started or unquiesced

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