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Date: Tue, 22 Apr 2025 14:20:18 +0200
From: Greg KH <gregkh@linuxfoundation.org>
To: Muchun Song <songmuchun@bytedance.com>
Cc: stable@vger.kernel.org, muchun.song@linux.dev,
	Ming Lei <ming.lei@redhat.com>, Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH 5.15.y] block: fix missing dispatching request when queue
 is started or unquiesced
Message-ID: <2025042244-knoll-defensive-0f43@gregkh>
References: <2024120323-snowiness-subway-3844@gregkh>
 <20250317071821.22449-1-songmuchun@bytedance.com>
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In-Reply-To: <20250317071821.22449-1-songmuchun@bytedance.com>

On Mon, Mar 17, 2025 at 03:18:21PM +0800, 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()
>                                                 virtblk_done()
>                         blk_mq_try_issue_directly()
>                           if (blk_mq_hctx_stopped())
>   blk_mq_request_bypass_insert()                  blk_mq_run_hw_queue()
>   blk_mq_run_hw_queue()     blk_mq_run_hw_queue()
>                             blk_mq_insert_request()
>                             return
> 
> 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.
> 
> Fixes: d964f04a8fde ("blk-mq: fix direct issue")
> Cc: stable@vger.kernel.org
> Cc: Muchun Song <muchun.song@linux.dev>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Reviewed-by: Ming Lei <ming.lei@redhat.com>
> Link: https://lore.kernel.org/r/20241014092934.53630-2-songmuchun@bytedance.com
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> (cherry picked from commit 2003ee8a9aa14d766b06088156978d53c2e9be3d)

This was NOT a clean cherry-pick, always document what you had to do
differently from the original change please.

Please fix this up and resend a new version.

thanks,

greg k-h