linux-kernel - Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU and generation based scheme

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Message-ID: <20171214202042.GG3326@worktop>
Date:   Thu, 14 Dec 2017 21:20:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Bart Van Assche <Bart.VanAssche@....com>
Cc:     "tj@...nel.org" <tj@...nel.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "kernel-team@...com" <kernel-team@...com>,
        "oleg@...hat.com" <oleg@...hat.com>, "hch@....de" <hch@....de>,
        "jianchao.w.wang@...cle.com" <jianchao.w.wang@...cle.com>,
        "osandov@...com" <osandov@...com>
Subject: Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU
 and generation based scheme

On Thu, Dec 14, 2017 at 06:51:11PM +0000, Bart Van Assche wrote:
> On Tue, 2017-12-12 at 11:01 -0800, Tejun Heo wrote:
> > +	write_seqcount_begin(&rq->gstate_seq);
> > +	blk_mq_rq_update_state(rq, MQ_RQ_IN_FLIGHT);
> > +	blk_add_timer(rq);
> > +	write_seqcount_end(&rq->gstate_seq);
> 
> My understanding is that both write_seqcount_begin() and write_seqcount_end()
> trigger a write memory barrier. Is a seqcount really faster than a spinlock?

Yes lots, no atomic operations and no waiting.

The only constraint for write_seqlock is that there must not be any
concurrency.

But now that I look at this again, TJ, why can't the below happen?

	write_seqlock_begin();
	blk_mq_rq_update_state(rq, IN_FLIGHT);
	blk_add_timer(rq);
	<timer-irq>
		read_seqcount_begin()
			while (seq & 1)
				cpurelax();
		// life-lock
	</timer-irq>
	write_seqlock_end();