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

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Message-ID: <20070107104328.GC13579@in.ibm.com>
Date:	Sun, 7 Jan 2007 16:13:28 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Oleg Nesterov <oleg@...sign.ru>
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 Sat, Jan 06, 2007 at 08:34:16PM +0300, Oleg Nesterov wrote:
> I suspect this can't help either.
> 
> The problem is that flush_workqueue() may be called while cpu hotplug event
> in progress and CPU_DEAD waits for kthread_stop(), so we have the same dead
> lock if work->func() does flush_workqueue(). This means that Andrew's change
> to use preempt_disable() is good and anyway needed.

Well ..a lock_cpu_hotplug() in run_workqueue() and support for recursive
calls to lock_cpu_hotplug() by the same thread will avoid the problem
you mention. This will need changes to task_struct to track the
recursion depth. Alternately this can be supported w/o changes to
task_struct by 'biasing' readers over writers as I believe Gautham's 
patches [1] do.

1. http://lkml.org/lkml/2006/10/26/65

-- 
Regards,
vatsa



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