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

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Message-ID: <20070109143823.GA89@tv-sign.ru>
Date:	Tue, 9 Jan 2007 17:38:23 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Srivatsa Vaddagiri <vatsa@...ibm.com>
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 01/09, Srivatsa Vaddagiri wrote:
>
> On Mon, Jan 08, 2007 at 08:06:35PM +0300, Oleg Nesterov wrote:
> > Ah, missed you point, thanks. Yet another old problem which was not introduced
> > by recent changes. And yet another indication we should avoid kthread_stop()
> > on CPU_DEAD event :) I believe this is easy to fix, but need to think more.
> 
> I think the problem is not just with CPU_DEAD. Anyone who calls
> cleanup_workqueue_thread (say destroy_workqueue?) will see this race. 

destroy_workqueue() first does flush_workqueue(), so it should be ok.

Anyway I agree with you, we shouldn't clear cwq->thread until it exits,

> Do you see any problems if cleanup_workqueue_thread is changed as:
> 
> cleanup_workqueue_thread()
> {
> 	kthread_stop(p);
> 	spin_lock(cwq->lock);
> 	cwq->thread = NULL;
> 	spin_unlock(cwq->lock);
> }

I think the same. In fact I suspect we even don't need spin_lock, but didn't
have a time to read the code since our discussion.

Oleg.

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