linux-kernel - Re: [PATCH] fix-flush_workqueue-vs-cpu

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Date:	Sun, 7 Jan 2007 20:09:32 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Srivatsa Vaddagiri <vatsa@...ibm.com>
Cc:	Andrew Morton <akpm@...l.org>, David Howells <dhowells@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...l.org>,
	linux-kernel@...r.kernel.org, Gautham shenoy <ego@...ibm.com>
Subject: Re: [PATCH] fix-flush_workqueue-vs-cpu_dead-race-update

On 01/07, Srivatsa Vaddagiri wrote:
>
> On Sun, Jan 07, 2007 at 03:56:03PM +0300, Oleg Nesterov wrote:
> > Srivatsa, I'm completely new to cpu-hotplug, so please correct me if I'm
> > wrong (in fact I _hope_ I am wrong) but as I see it, the hotplug/workqueue
> > interaction is broken by design, it can't be fixed by changing just locking.
> > 
> > Once again. CPU dies, CPU_DEAD calls kthread_stop() and sleeps until
> > cwq->thread exits. To do so, this thread must at least complete the
> > currently running work->func().
> 
> If run_workqueue() takes a lock_cpu_hotplug() successfully, then we shouldnt 
> even reach till this point, as it will block writers (cpu_down/up) until it
> completes.
> 
> 
> 	run_workqueue()
> 	---------------
> 	
> try_again:
> 	rc = lock_cpu_hotplug_interruptible();
> 	
> 	if (rc && kthread_should_stop())
> 		return;
> 	
> 	if (rc != 0)
> 		goto try_again;
> 	
> 	/* cpu_down/up shouldnt happen now untill we call unlock_cpu_hotplug */
> 	while (!list_empty(..))
> 		work->func();

This mean that every work->func() which may sleep delays cpu_down/up unpredictable,
not good. What about work->func which sleeps then re-queues itself? I guess we can
solve this, but this is what I said "other changes".

Also, lock_cpu_hotplug() should be per-cpu, otherwise we have livelock.

Not that I am against lock_cpu_hotplug (I can't judge), but its usage in run_workqueue
looks like complication to me. I may be wrong. But the main problem we don't have it :)

Oleg.

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