Hello, Peter.

On Thu, Dec 14, 2017 at 09:20:42PM +0100, Peter Zijlstra wrote:

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();

Ah, you're right.  For both gstate_seq and aborted_gstate_sync, we can
push all synchronization to the timeout side - ie. gstate_seq read can
yield, pause or synchronize_rcu and hte aborted_gstate_sync can
disable irq around update.

Thanks.

-- 
tejun