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

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Message-Id: <476C2322-BD02-4631-9588-E182847F9EC6@linux.dev>
Date: Wed, 11 Sep 2024 10:43:56 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Jens Axboe <axboe@...nel.dk>
Cc: Muchun Song <songmuchun@...edance.com>,
 Ming Lei <ming.lei@...hat.com>,
 Yu Kuai <yukuai1@...weicloud.com>,
 "open list:BLOCK LAYER" <linux-block@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>,
 stable@...r.kernel.org
Subject: Re: [PATCH v2 1/3] block: fix missing dispatching request when queue
 is started or unquiesced



> On Sep 10, 2024, at 21:17, Jens Axboe <axboe@...nel.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