linux-kernel - Re: [PATCH 2/8] blk-mq: protect completion path with RCU

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Message-ID: <b4fdf1c4-2728-5537-a00f-09db0a4b25e4@suse.de>
Date:   Tue, 9 Jan 2018 08:08:04 +0100
From:   Hannes Reinecke <hare@...e.de>
To:     Tejun Heo <tj@...nel.org>, jack@...e.cz, axboe@...nel.dk,
        clm@...com, jbacik@...com
Cc:     kernel-team@...com, linux-kernel@...r.kernel.org,
        linux-btrfs@...r.kernel.org, peterz@...radead.org,
        jianchao.w.wang@...cle.com, Bart.VanAssche@....com,
        linux-block@...r.kernel.org
Subject: Re: [PATCH 2/8] blk-mq: protect completion path with RCU

On 01/08/2018 08:15 PM, Tejun Heo wrote:
> Currently, blk-mq protects only the issue path with RCU.  This patch
> puts the completion path under the same RCU protection.  This will be
> used to synchronize issue/completion against timeout by later patches,
> which will also add the comments.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> ---
>  block/blk-mq.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index ddc9261..6741c3e 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -584,11 +584,16 @@ static void hctx_lock(struct blk_mq_hw_ctx *hctx, int *srcu_idx)
>  void blk_mq_complete_request(struct request *rq)
>  {
>  	struct request_queue *q = rq->q;
> +	struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, rq->mq_ctx->cpu);
> +	int srcu_idx;
>  
>  	if (unlikely(blk_should_fake_timeout(q)))
>  		return;
> +
> +	hctx_lock(hctx, &srcu_idx);
>  	if (!blk_mark_rq_complete(rq))
>  		__blk_mq_complete_request(rq);
> +	hctx_unlock(hctx, srcu_idx);
>  }
>  EXPORT_SYMBOL(blk_mq_complete_request);
>  
> 
Hmm. Why do we need to call blk_mq_map_queue() here?
Is there a chance that we end up with a _different_ hctx on completion
than that one used for submission?
If not, why can't we just keep a pointer to the hctx in struct request?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@...e.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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