linux-kernel - Re: [PATCH] flush_cpu_workqueue: don't flush an empty ->worklist

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Message-ID: <20070109150755.GB89@tv-sign.ru>
Date:	Tue, 9 Jan 2007 18:07:55 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Andrew Morton <akpm@...l.org>
Cc:	vatsa@...ibm.com, 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] flush_cpu_workqueue: don't flush an empty ->worklist

On 01/08, Andrew Morton wrote:
>
> That's not correct.  freeze_processes() will freeze *all* processes.  All
> of them are forced to enter refrigerator().  With the mysterious exception
> of some I/O-related kernel threads, which might need some thought.

Yes, and we can remove a dead CPU safely, this part is ok.

> > and 
> > http://marc.theaimsgroup.com/?l=linux-kernel&m=116817460726058.
> 
> Am not sure how that's related.

but at some point we should thaw processes, including cwq->thread which
should die. So we are doing things like take_over_work() and this is the
source of races, because the dead CPU is not on cpu_online_map.

flush_workqueue() doesn't use any locks now. If we use freezer to implement
cpu-hotplug nothing will change, we still have races.

It looks so obvious to me, so I'm starting to suspect I missed something
very simple :(

Oleg.

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