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Date:   Mon, 8 Jun 2020 08:45:57 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Jens Axboe <axboe@kernel.dk>, frederic@kernel.org,
        linux-kernel@vger.kernel.org, linux-block@vger.kernel.org
Subject: Re: blk-softirq vs smp_call_function_single_async()
Message-ID: <20200608154557.GA26611@infradead.org>
References: <20200608115800.GA2531@hirez.programming.kicks-ass.net>
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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.