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();
