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Date:   Mon, 8 Jun 2020 18:40:09 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Christoph Hellwig <hch@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()
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On Mon, Jun 08, 2020 at 09:33:42AM -0700, Christoph Hellwig wrote:
> 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?

There isn't one, it was meant to be used with static allocations.

Frederic proposed changing all these to irq_work, and I think I'll go do
that. First dinner though.