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Date:   Fri, 15 Dec 2017 03:39:30 +0100
From:   Mike Galbraith <efault@....de>
To:     Peter Zijlstra <peterz@...radead.org>,
        Bart Van Assche <Bart.VanAssche@....com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "kernel-team@...com" <kernel-team@...com>,
        "oleg@...hat.com" <oleg@...hat.com>, "hch@....de" <hch@....de>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "jianchao.w.wang@...cle.com" <jianchao.w.wang@...cle.com>,
        "osandov@...com" <osandov@...com>, "tj@...nel.org" <tj@...nel.org>
Subject: Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU
 and generation based scheme

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:
> 
> > 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

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