On Thu, 2017-12-14 at 22:54 +0100, Peter Zijlstra wrote:

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

quoted

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();


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.

Most kthreads, including kworkers, are very frequently SCHED_FIFO here.

	-Mike