linux-kernel - Re: Problem with freezable workqueues

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Message-ID: <20070228030726.GK29179@in.ibm.com>
Date:	Wed, 28 Feb 2007 08:37:26 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Oleg Nesterov <oleg@...sign.ru>, Pavel Machek <pavel@....cz>,
	Gautham R Shenoy <ego@...ibm.com>,
	Johannes Berg <johannes@...solutions.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Problem with freezable workqueues

On Wed, Feb 28, 2007 at 12:57:35AM +0100, Rafael J. Wysocki wrote:
> > How about other kthread_stop()s ? For example, kernel/softirq.c:cpu_callback() ?
> 
> They all are PF_NOFREEZE, I suppose.  If we make all workqueues nonfreezable
> (as they were before), the problem won't appear.

We can just thaw the worker thread selectively before kthread_stopping
them. This will let us freeze all worker threads (which we want to for
hotplug anyway).

> > I think we need a general "cpu_down() after freeze" implementation, this is what
> > Gautham and Srivatsa are working on, right?
> 
> Yes, certainly.

Hmm ..good point. So can we assume that disable/enable_nonboot_cpus() are called
with processes frozen already?

Gautham, you need to take this into account in your patchset!

> > I'm afraid this is racy. We can't touch *cwq, it may be freed. Suppose
> > that another thread does destroy_workqueue(), and we thaw that thread
> > before cwq->thread.
> 
> Okay, in that case I'd suggest removing create_freezeable_workqueue() and
> make all workqueues nonfreezable once again for 2.6.21 (as far as I know, only
> the two XFS workqueues are affected).

See above suggestion of thawing worker thread before kthread_stopping
it.

-- 
Regards,
vatsa
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