linux-kernel - Re: Re: [PATCH 4/4] blk-mq: improve readability of blk_mq_alloc

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Message-ID: <3caa5abc.16387a.1843622d8cf.Coremail.nickyc975@zju.edu.cn>
Date:   Wed, 2 Nov 2022 10:19:24 +0800 (GMT+08:00)
From:   "Jinlong Chen" <nickyc975@....edu.cn>
To:     "Christoph Hellwig" <hch@....de>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH 4/4] blk-mq: improve readability of
 blk_mq_alloc_request()

> On Tue, Nov 01, 2022 at 11:11:37PM +0800, Jinlong Chen wrote:
> > Add a helper blk_mq_alloc_request_nocache() to alloc request without
> > cache. This makes blk_mq_alloc_request() more readable.
> > 
> > Signed-off-by: Jinlong Chen <nickyc975@....edu.cn>
> > ---
> >  block/blk-mq.c | 47 +++++++++++++++++++++++++++++------------------
> >  1 file changed, 29 insertions(+), 18 deletions(-)
> > 
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 87a6348a0d0a..2fae111a42c8 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -572,36 +572,47 @@ static struct request *blk_mq_alloc_cached_request(struct request_queue *q,
> >  	return rq;
> >  }
> >  
> > +static struct request *blk_mq_alloc_request_nocache(struct request_queue *q,
> > +		blk_opf_t opf, blk_mq_req_flags_t flags)
> 
> The name is a bit odd, but I can't think off a better one.
> 
> > +	struct blk_mq_alloc_data data = {
> >  			.q		= q,
> >  			.flags		= flags,
> >  			.cmd_flags	= opf,
> >  			.nr_tags	= 1,
> >  		};
> 
> And this now has superflous indenation.  Overall, while the separate
> helper looks marginally nicer, I'm not really sure it is worth the
> churn.

I'll drop the patch if you think it is not worth the churn. But I
started doing this because of the following goto statement:

	rq = blk_mq_alloc_cached_request(q, opf, flags);
	if (!rq) {
		[...]
		ret = blk_queue_enter(q, flags);
		[...]
		rq = __blk_mq_alloc_requests(&data);
		if (!rq)
			goto out_queue_exit;
	}
	[...]
out_queue_exit:
	blk_queue_exit(q);
	return ERR_PTR(-EWOULDBLOCK);

Queue entering has been moved into the fallback path, left queue exiting
outside. Should I just eliminate the goto statement and move the error
handling into the fallback path too? Like:

	rq = blk_mq_alloc_cached_request(q, opf, flags);
	if (!rq) {
		[...]
		ret = blk_queue_enter(q, flags);
		[...]
		rq = __blk_mq_alloc_requests(&data);
		if (!rq) {
			blk_queue_exit(q);
			return ERR_PTR(-EWOULDBLOCK);
		}
	}

Thanks!
Jinlong Chen