linux-kernel - Re: [PATCH 3/3] blk-mq: Use llist_head for blk_cpu

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Message-ID: <20210125083903.GB945284@infradead.org>
Date:   Mon, 25 Jan 2021 08:39:03 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Christoph Hellwig <hch@...radead.org>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 3/3] blk-mq: Use llist_head for blk_cpu_done

On Mon, Jan 25, 2021 at 09:32:04AM +0100, Sebastian Andrzej Siewior wrote:
> On 2021-01-25 08:30:12 [+0000], Christoph Hellwig wrote:
> > > +static void blk_mq_complete_send_ipi(struct request *rq)
> > > +{
> > > +	struct llist_head *list;
> > > +	unsigned int cpu;
> > > +
> > > +	cpu = rq->mq_ctx->cpu;
> > > +	list = &per_cpu(blk_cpu_done, cpu);
> > > +	if (llist_add(&rq->ipi_list, list)) {
> > > +		INIT_CSD(&rq->csd, __blk_mq_complete_request_remote, rq);
> > > +		smp_call_function_single_async(cpu, &rq->csd);
> > > +	}
> > > +}
> > 
> > Nit: it would be nice to initialize cpu and list in the declaration
> > lines.
> 
> Why? They get initialized later.

Because:

	unsigned int cpu = rq->mq_ctx->cpu;
	struct llist_head *list = &per_cpu(blk_cpu_done, cpu);

is a lot easier to follow than:

	struct llist_head *list;
	unsigned int cpu;

	cpu = rq->mq_ctx->cpu;
	list = &per_cpu(blk_cpu_done, cpu);