linux-kernel - Re: [PATCH, RFC] reimplement flush

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Message-ID: <20070104155649.GB27603@in.ibm.com>
Date:	Thu, 4 Jan 2007 21:26:49 +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, RFC] reimplement flush_workqueue()

On Thu, Jan 04, 2007 at 05:29:36PM +0300, Oleg Nesterov wrote:
> Thanks, I need to think about this.
> 
> However I am not sure I fully understand the problem.
> 
> First, this deadlock was not introduced by recent changes (including "single
> threaded flush_workqueue() takes workqueue_mutex too"), yes?

AFAIK this deadlock originated from Andrew's patch here:

	http://lkml.org/lkml/2006/12/7/231

(Yes, your patches didnt introduce this. I was just reiterating here my
earlier point that workqueue code is broken of late wrt cpu hotplug).

> Also, it seems to me we have a much more simple scenario for deadlock.
> 
> events/0 runs run_workqueue(), work->func() sleeps or takes a preemtion. CPU 0
> dies, keventd thread migrates to another CPU. CPU_DEAD calls kthread_stop() under
> workqueue_mutex and waits for until kevents thread exits. Now, if this work (or
> another work pending on cwq->worklist) takes workqueue_mutex (for example, does
> flush_workqueue) we have a deadlock.
> 
> No?

Yes, the above scenario also will cause a deadlock. 

I supposed one could avoid the deadlock by having a 'workqueue_mutex_held' 
flag and avoid taking the mutex set under some conditions, but IMHO a
more neater solution is to provide a cpu-hotplug lock which works under
all these corner cases. One such proposal was made here:

	http://lkml.org/lkml/2006/10/26/65
	
-- 
Regards,
vatsa
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