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

[Peter Zijlstra]

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();