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

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Date:   Fri, 15 Dec 2017 08:31:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "jianchao.wang" <jianchao.w.wang@...cle.com>
Cc:     Bart Van Assche <Bart.VanAssche@....com>,
        "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>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "osandov@...com" <osandov@...com>, "tj@...nel.org" <tj@...nel.org>
Subject: Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU
 and generation based scheme

On Fri, Dec 15, 2017 at 10:12:50AM +0800, jianchao.wang wrote:
> > That only makes it a little better:
> > 
> > 	Task-A					Worker
> > 
> > 	write_seqcount_begin()
> > 	blk_mq_rw_update_state(rq, IN_FLIGHT)
> > 	blk_add_timer(rq)
> > 	<timer>
> > 		schedule_work()
> > 	</timer>
> > 	<context-switch to worker>
> > 						read_seqcount_begin()
> > 							while(seq & 1)
> > 								cpu_relax();
> > 
> Hi Peter
> 
> The current seqcount read side is as below:
> 	do {
> 		start = read_seqcount_begin(&rq->gstate_seq);


static inline unsigned read_seqcount_begin(const seqcount_t *s)
{
	seqcount_lockdep_reader_access(s);
	return raw_read_seqcount_begin(s);
}

static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
{
	unsigned ret = __read_seqcount_begin(s);
	smp_rmb();
	return ret;
}

static inline unsigned __read_seqcount_begin(const seqcount_t *s)
{
	unsigned ret;

repeat:
	ret = READ_ONCE(s->sequence);
	if (unlikely(ret & 1)) {
		cpu_relax();
		goto repeat;
	}
	return ret;
}