Re: [PATCH v2 2/3] block: fix ordering between checking QUEUE_FLAG_QUIESCED and adding requests

[Jens Axboe]

On 9/3/24 2:16 AM, Muchun Song wrote:
> Supposing the following scenario.
> 
> CPU0                                        CPU1
> 
> blk_mq_insert_request()         1) store    blk_mq_unquiesce_queue()
> blk_mq_run_hw_queue()                       blk_queue_flag_clear(QUEUE_FLAG_QUIESCED)       3) store
>     if (blk_queue_quiesced())   2) load         blk_mq_run_hw_queues()
>         return                                      blk_mq_run_hw_queue()
>     blk_mq_sched_dispatch_requests()                    if (!blk_mq_hctx_has_pending())     4) load
>                                                            return
> 
> The full memory barrier should be inserted between 1) and 2), as well as
> between 3) and 4) to make sure that either CPU0 sees QUEUE_FLAG_QUIESCED is
> cleared or CPU1 sees dispatch list or setting of bitmap of software queue.
> Otherwise, either CPU will not re-run the hardware queue causing starvation.
> 
> So the first solution is to 1) add a pair of memory barrier to fix the
> problem, another solution is to 2) use hctx->queue->queue_lock to synchronize
> QUEUE_FLAG_QUIESCED. Here, we chose 2) to fix it since memory barrier is not
> easy to be maintained.

Same comment here, 72-74 chars wide please.

> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index b2d0f22de0c7f..ac39f2a346a52 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2202,6 +2202,24 @@ void blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, unsigned long msecs)
>  }
>  EXPORT_SYMBOL(blk_mq_delay_run_hw_queue);
>  
> +static inline bool blk_mq_hw_queue_need_run(struct blk_mq_hw_ctx *hctx)
> +{
> +	bool need_run;
> +
> +	/*
> +	 * When queue is quiesced, we may be switching io scheduler, or
> +	 * updating nr_hw_queues, or other things, and we can't run queue
> +	 * any more, even blk_mq_hctx_has_pending() can't be called safely.
> +	 *
> +	 * And queue will be rerun in blk_mq_unquiesce_queue() if it is
> +	 * quiesced.
> +	 */
> +	__blk_mq_run_dispatch_ops(hctx->queue, false,
> +				  need_run = !blk_queue_quiesced(hctx->queue) &&
> +					      blk_mq_hctx_has_pending(hctx));
> +	return need_run;
> +}

This __blk_mq_run_dispatch_ops() is also way too wide, why didn't you
just break it like where you copied it from?

> +
>  /**
>   * blk_mq_run_hw_queue - Start to run a hardware queue.
>   * @hctx: Pointer to the hardware queue to run.
> @@ -2222,20 +2240,23 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
>  
>  	might_sleep_if(!async && hctx->flags & BLK_MQ_F_BLOCKING);
>  
> -	/*
> -	 * When queue is quiesced, we may be switching io scheduler, or
> -	 * updating nr_hw_queues, or other things, and we can't run queue
> -	 * any more, even __blk_mq_hctx_has_pending() can't be called safely.
> -	 *
> -	 * And queue will be rerun in blk_mq_unquiesce_queue() if it is
> -	 * quiesced.
> -	 */
> -	__blk_mq_run_dispatch_ops(hctx->queue, false,
> -		need_run = !blk_queue_quiesced(hctx->queue) &&
> -		blk_mq_hctx_has_pending(hctx));
> +	need_run = blk_mq_hw_queue_need_run(hctx);
> +	if (!need_run) {
> +		unsigned long flags;
>  
> -	if (!need_run)
> -		return;
> +		/*
> +		 * synchronize with blk_mq_unquiesce_queue(), becuase we check
> +		 * if hw queue is quiesced locklessly above, we need the use
> +		 * ->queue_lock to make sure we see the up-to-date status to
> +		 * not miss rerunning the hw queue.
> +		 */
> +		spin_lock_irqsave(&hctx->queue->queue_lock, flags);
> +		need_run = blk_mq_hw_queue_need_run(hctx);
> +		spin_unlock_irqrestore(&hctx->queue->queue_lock, flags);
> +
> +		if (!need_run)
> +			return;
> +	}

Is this not solvable on the unquiesce side instead? It's rather a shame
to add overhead to the fast path to avoid a race with something that's
super unlikely, like quisce.

-- 
Jens Axboe