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

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

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;
}