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

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Date:	Sun, 7 Jan 2007 11:59:57 -0800
From:	Andrew Morton <akpm@...l.org>
To:	vatsa@...ibm.com
Cc:	Oleg Nesterov <oleg@...sign.ru>,
	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 Sun, 7 Jan 2007 16:30:13 +0530
Srivatsa Vaddagiri <vatsa@...ibm.com> wrote:

> On Sat, Jan 06, 2007 at 11:11:17AM -0800, Andrew Morton wrote:
> > Has anyone thought seriously about using the process freezer in the
> > cpu-down/cpu-up paths?  That way we don't need to lock anything anywhere?
> 
> How would this provide a stable access to cpu_online_map in functions
> that need to block while accessing it (as flush_workqueue requires)?

If a thread simply blocks, that will not permit a cpu plug/unplug to proceed.

The thread had to explicitly call try_to_freeze().  CPU plug/unplug will
not occur (and cpu_online_map will not change) until every process in the
machine has called try_to_freeze()).

So the problem which you're referring to will only occur if a workqueue
callback function calls try_to_freeze(), which would be mad.

Plus flush_workqueue() is on the way out.  We're slowly edging towards a
working cancel_work() which will only block if the work which you're trying
to cancel is presently running.  With that, pretty much all the
flush_workqueue() calls go away, and all these accidental rarely-occurring
deadlocks go away too.

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