linux-kernel - Re: [PATCH 2/4] workqueue: don't check wq->rescuer in rescuer

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Message-ID: <20200529150434.GF3530656@mtj.duckdns.org>
Date:   Fri, 29 May 2020 11:04:34 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] workqueue: don't check wq->rescuer in rescuer

Hello,

On Fri, May 29, 2020 at 10:58:46PM +0800, Lai Jiangshan wrote:
> I'm not sure I understood your words. And I'm not
> sure which function may use freed object in "use-after-free".
> Is it "send_mayday() may use a freed rescuer"?
> 
> This patch relies on
> def98c84b6 ("workqueue: Fix spurious sanity check failures in
> destroy_workqueue()")
> to move the kthread_stop() before the sanity check and the work
> of drain_workqueue() which guarantees there is no work item
> in the workqueue. If send_mayday() still goes wrong after
> drain_workqueue(), the user must have queued work items and
> invoked destroy_workqueue() concurrently. It is excellent
> if the sanity check can find this case out, but it is not possible
> that the sanity check can always live through it since it is
> not worqueue's internal fault. We hope the sanity check can
> find all the internal fault, but not to the extend that
> it can always work when any user uses it in a very wrong way.

Yeah, it's not fool proof but it's difficult for me to see what is better
after the patch. What does the patch actually improve?

Thanks.

-- 
tejun