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Date:   Fri, 5 May 2023 07:56:26 +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,
        Jaegeuk Kim <jaegeuk@kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        Damien Le Moal <damien.lemoal@opensource.wdc.com>,
        Ming Lei <ming.lei@redhat.com>
Subject: Re: [PATCH v4 08/11] block: mq-deadline: Reduce lock contention
Message-ID: <20230505055626.GD11748@lst.de>
References: <20230503225208.2439206-1-bvanassche@acm.org> <20230503225208.2439206-9-bvanassche@acm.org>
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On Wed, May 03, 2023 at 03:52:05PM -0700, Bart Van Assche wrote:
> blk_mq_free_requests() calls dd_finish_request() indirectly. Prevent
> nested locking of dd->lock and dd->zone_lock by unlocking dd->lock
> before calling blk_mq_free_requests().

Do you have a reproducer for this that we could wire up in blktests?
Also please add a Fixes tag and move it to the beginning of the series.

>  static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
>  			      blk_insert_t flags)
> +	__must_hold(dd->lock)
>  {
>  	struct request_queue *q = hctx->queue;
>  	struct deadline_data *dd = q->elevator->elevator_data;
> @@ -784,7 +785,9 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
>  	}
>  
>  	if (blk_mq_sched_try_insert_merge(q, rq, &free)) {
> +		spin_unlock(&dd->lock);
>  		blk_mq_free_requests(&free);
> +		spin_lock(&dd->lock);
>  		return;

Fiven that free is a list, why don't we declare the free list in
dd_insert_requests and just pass it to dd_insert_request and then do
one single blk_mq_free_requests call after the loop?