linux-kernel - Re: [PATCH] blk-mq: Put driver tag in blk_mq_dispatch_rq

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Message-ID: <e5416179-2ba0-c9a8-1b86-d52eae29e146@acm.org>
Date:   Fri, 17 Apr 2020 19:43:14 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     John Garry <john.garry@...wei.com>, axboe@...nel.dk
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        ming.lei@...hat.com
Subject: Re: [PATCH] blk-mq: Put driver tag in blk_mq_dispatch_rq_list() when
 no budget

On 2020-04-16 04:18, John Garry wrote:
> If in blk_mq_dispatch_rq_list() we find no budget, then we break of the
> dispatch loop, but the request may keep the driver tag, evaulated
> in 'nxt' in the previous loop iteration.
> 
> Fix by putting the driver tag for that request.
> 
> Signed-off-by: John Garry <john.garry@...wei.com>
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 8e56884fd2e9..a7785df2c944 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1222,8 +1222,10 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
>  		rq = list_first_entry(list, struct request, queuelist);
>  
>  		hctx = rq->mq_hctx;
> -		if (!got_budget && !blk_mq_get_dispatch_budget(hctx))
> +		if (!got_budget && !blk_mq_get_dispatch_budget(hctx)) {
> +			blk_mq_put_driver_tag(rq);
>  			break;
> +		}
>  
>  		if (!blk_mq_get_driver_tag(rq)) {
>  			/*

Is this something that can only happen if q->mq_ops->queue_rq(hctx, &bd)
returns another value than BLK_STS_OK, BLK_STS_RESOURCE and
BLK_STS_DEV_RESOURCE? If so, please add a comment in the source code
that explains this.

Is this perhaps a bug fix for 0bca799b9280 ("blk-mq: order getting
budget and driver tag")? If so, please mention this and add Cc tags for
the people who were Cc-ed on that patch.

Thanks,

Bart.