Re: [PATCH 5.15.y] block: fix missing dispatching request when queue is started or unquiesced

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Mar 17, 2025 at 11:30:39AM +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>
> ---
>  block/blk-mq.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 46cb802cfcf05..a15c665a77100 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2048,7 +2048,6 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
>  	 * and avoid driver to try to dispatch again.
>  	 */
>  	if (blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)) {
> -		run_queue = false;
>  		bypass_insert = false;
>  		goto insert;
>  	}
> -- 
> 2.20.1
> 
> 

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>