Re: [PATCH v6 1/1] block/blk-mq: use atomic_t for quiesce_depth to avoid lock contention on RT

[Bart Van Assche <bvanassche@acm.org>]

On 5/6/26 8:56 AM, Ionut Nechita (Wind River) wrote:
>   void blk_mq_quiesce_queue_nowait(struct request_queue *q)
>   {
> -	unsigned long flags;
> -
> -	spin_lock_irqsave(&q->queue_lock, flags);
> -	if (!q->quiesce_depth++)
> -		blk_queue_flag_set(QUEUE_FLAG_QUIESCED, q);
> -	spin_unlock_irqrestore(&q->queue_lock, flags);
> +	atomic_inc(&q->quiesce_depth);
> +	/*
> +	 * Pairs with smp_rmb() in blk_mq_run_hw_queue(): make the
> +	 * incremented quiesce_depth observable to readers re-checking
> +	 * the quiesce state, so they don't dispatch on a quiesced queue.
> +	 */
> +	smp_mb__after_atomic();
>   }

No, this is not sufficient to guarantee that blk_mq_run_hw_queue() sees
the latest value of q->quiesce_depth. If you want to achieve that I
think the only option is to protect the atomic_inc() above with
hctx->queue->queue_lock.

> @@ -2362,17 +2365,15 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
>   
>   	need_run = blk_mq_hw_queue_need_run(hctx);
>   	if (!need_run) {
> -		unsigned long flags;
> -
>   		/*
> -		 * Synchronize with blk_mq_unquiesce_queue(), because 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.
> +		 * Re-check the quiesce state after a read barrier. Pairs with
> +		 * smp_mb__after_atomic() in blk_mq_quiesce_queue_nowait() and
> +		 * blk_mq_unquiesce_queue() so we don't miss rerunning the hw
> +		 * queue when a concurrent unquiesce has just dropped the
> +		 * quiesce_depth to zero.
>   		 */
> -		spin_lock_irqsave(&hctx->queue->queue_lock, flags);
> +		smp_rmb();
>   		need_run = blk_mq_hw_queue_need_run(hctx);
> -		spin_unlock_irqrestore(&hctx->queue->queue_lock, flags);

If the atomic_inc() in blk_mq_quiesce_queue_nowait() is protected by
hctx->queue->queue_lock then the above code doesn't have to be modified.

Thanks,

Bart.