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

[Peter Zijlstra <peterz@infradead.org>]

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();