linux-kernel - Re: blk-softirq vs smp_call_function_single

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Date:   Mon, 8 Jun 2020 09:33:42 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 05:58:33PM +0200, Peter Zijlstra wrote:
> > A request can only be completed once.
> 
> Sure, but that doesn't help.
> 
> CPU0				CPU1
> 
>  raise_blk_irq()		BLOCK_SOFTIRQ
>    IPI -> CPU1
> 
> 				// picks up thing from CPU0
> 				req->complete(req);
> 
> 
> 	<big hole where CSD is active and request completed>
> 
> 				<IPI>
> 				  trigger_softirq()
> 
> 
> What happens to a struct request after completion, is it free()d
> or reused? If reused, how do we guarantee CSD completion before
> free()ing?

The request is freed in the block layer sense by __blk_mq_free_request
(which doesn't actually free the memory, so it eventually gets reused).

__blk_mq_free_request is called from blk_mq_end_request which
is called from the drivers ->complete_rq method, which is called
from the block softirq.

What is the method to guaranteed CSD completion?