On 12/15/2017 05:54 AM, Peter Zijlstra wrote:

On Thu, Dec 14, 2017 at 09:42:48PM +0000, Bart Van Assche wrote:

quoted

On Thu, 2017-12-14 at 21:20 +0100, Peter Zijlstra wrote:

quoted

On Thu, Dec 14, 2017 at 06:51:11PM +0000, Bart Van Assche wrote:

quoted

On Tue, 2017-12-12 at 11:01 -0800, Tejun Heo wrote:

quoted

+	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();

Hello Peter,

Some time ago the block layer was changed to handle timeouts in thread context
instead of interrupt context. See also commit 287922eb0b18 ("block: defer
timeouts to a workqueue").

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);
		gstate = READ_ONCE(rq->gstate);
		deadline = rq->deadline;
	} while (read_seqcount_retry(&rq->gstate_seq, start));
read_seqcount_retry() doesn't check the bit 0, but whether the saved value from 
read_seqcount_begin() is equal to the current value of seqcount.
pls refer:
static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
{
	return unlikely(s->sequence != start);
}

Thanks
Jianchao

Now normally this isn't fatal because Worker will simply spin its entire
time slice away and we'll eventually schedule our Task-A back in, which
will complete the seqcount and things will work.

But if, for some reason, our Worker was to have RT priority higher than
our Task-A we'd be up some creek without no paddles.

We don't happen to have preemption of IRQs off here? That would fix
things nicely.