linux-kernel - Re: [patch, minor] workqueue: consistently use 'err' in __create_workqueue

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Date:	Tue, 29 Jul 2008 17:44:57 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Dmitry Adamushko <dmitry.adamushko@...il.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch, minor] workqueue: consistently use 'err' in __create_workqueue_key()

On 07/29, Oleg Nesterov wrote:
>
> On 07/29, Dmitry Adamushko wrote:
> >
> > And I'd say this behavior (of having a partially-created object
> > visible to the outside world) is not that robust. e.g. the
> > aforementioned race would be eliminated if we place a wq on the global
> > list only when it's been successfully initialized.
>
> Yes, we can change __create_workqueue_key() to check err == 0 before
> list_add(),

Well no, we can't do even this.

Then we have another race with cpu-hotplug. Suppose we have CPUs 0, 1, 2.
create_workqueue() fails to create cwq->thread for CPU 2 and calls
destroy_workqueue(). Before it takes the cpu_add_remove_lock, _cpu_down()
removes CPU 1 from cpu_populated_map, but since we didn't add this wq
on the global list, cwq[1]->thread remains alive.

destroy_workqueue() takes cpu_add_remove_lock, and calls
cleanup_workqueue_thread() for CPUs 0 and 2. cwq[1]->thread is lost.

Damn. I had this in mind when I wrote the code, but forgot. We need
comments, I'll send the patch.

Oleg.

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