On 12/15/2017 03:31 PM, Peter Zijlstra wrote:

On Fri, Dec 15, 2017 at 10:12:50AM +0800, jianchao.wang wrote:

quoted

quoted

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;
}

Really thanks for kindly pointing out.
jianchao