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Date:   Thu, 20 Jul 2023 07:54:37 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Bart Van Assche <bvanassche@acm.org>
Cc:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        Christoph Hellwig <hch@lst.de>,
        "Martin K . Petersen" <martin.petersen@oracle.com>,
        Ming Lei <ming.lei@redhat.com>,
        "James E.J. Bottomley" <jejb@linux.ibm.com>
Subject: Re: [PATCH v2 3/3] block: Improve performance for
 BLK_MQ_F_BLOCKING drivers
Message-ID: <20230720055437.GA2665@lst.de>
References: <20230719182243.2810134-1-bvanassche@acm.org> <20230719182243.2810134-4-bvanassche@acm.org>
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On Wed, Jul 19, 2023 at 11:22:42AM -0700, Bart Van Assche wrote:
> blk_mq_run_queue() runs the queue asynchronously if BLK_MQ_F_BLOCKING
> has been set.

Maybe something like:

blk_mq_run_queue() always runs the queue asynchronously if
BLK_MQ_F_BLOCKING is set on the tag_set.

> + *    for execution. Don't wait for completion. May sleep if BLK_MQ_F_BLOCKING
> + *    has been set.
>   *
>   * Note:
>   *    This function will invoke @done directly if the queue is dead.
> @@ -2213,6 +2214,8 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
>  	 */
>  	WARN_ON_ONCE(!async && in_interrupt());
>  
> +	might_sleep_if(!async && hctx->flags & BLK_MQ_F_BLOCKING);

This is some odd an very complex calling conventions.  I suspect most
!BLK_MQ_F_BLOCKING could also deal with the may sleep if not async,
and that would give us a much easier to audit change as we could
remove the WARN_ON_ONCE above and just do a:

	might_sleep_if(!async);

In fact this might be a good time to split up blk_mq_run_hw_queue
into blk_mq_run_hw_queue and blk_mq_run_hw_queue_async and do
away with the bool and have cristal clear calling conventions.

If we really need !async calles than can sleep we can add a specific
blk_mq_run_hw_queue_atomic.