linux-kernel - Re: blk-softirq vs smp_call_function_single

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Message-ID: <20200608154557.GA26611@infradead.org>
Date:   Mon, 8 Jun 2020 08:45:57 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jens Axboe <axboe@...nel.dk>, frederic@...nel.org,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: blk-softirq vs smp_call_function_single_async()

On Mon, Jun 08, 2020 at 01:58:00PM +0200, Peter Zijlstra wrote:
> Hi Jens,
> 
> I've been going through smp_call_function_single_async() users and
> stumbled upon blk-softirq.c, which has:
> 
> static int raise_blk_irq(int cpu, struct request *rq)
> {
> 	if (cpu_online(cpu)) {
> 		call_single_data_t *data = &rq->csd;
> 
> 		data->func = trigger_softirq;
> 		data->info = rq;
> 		data->flags = 0;
> 
> 		smp_call_function_single_async(cpu, data);
> 		return 0;
> 	}
> 
> 	return 1;
> }
> 
> What, if anything, guarantees rq->csd is not already in use at that
> time?

A request can only be completed once.