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Date:   Mon, 12 Dec 2022 12:27:20 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     Richard Clark <richard.xnu.clark@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] workqueue: Prevent a new work item from queueing into a
 destruction wq

On Mon, Dec 12, 2022 at 02:48:25PM +0800, Lai Jiangshan wrote:
> On Mon, Dec 12, 2022 at 2:23 PM Tejun Heo <tj@...nel.org> wrote:
> >
> > On Mon, Dec 12, 2022 at 02:18:36PM +0800, Richard Clark wrote:
> > > Currently the __WQ_DRAINING is used to prevent a new work item from queueing
> > > to a draining workqueue, but this flag will be cleared before the end of a
> > > RCU grace period. Because the workqueue instance is actually freed after
> > > the RCU grace period, this fact results in an opening window in which a new
> > > work item can be queued into a destorying workqueue and be scheduled
> > > consequently, for instance, the below code snippet demos this accident:
> >
> > I mean, this is just use-after-free. The same scenario can happen with
> > non-RCU frees or if there happens to be an RCU grace period inbetween. I'm
> > not sure what's being protected here.
> 
> I think it is a kind of debugging facility with no overhead in the
> fast path.
> 
> It is indeed the caller's responsibility not to do use-after-free.
> 
> For non-RCU free, the freed workqueue's state can be arbitrary soon and
> the caller might get a complaint. And if there are some kinds of debugging
> facilities for freed memory, the system can notice the problem earlier.
> 
> But now is RCU free for the workqueue, and the workqueue has nothing
> different between before and after destroy_workqueue() unless the
> grace period ends and the memory-allocation subsystem takes charge of
> the memory.

idk, maybe? It seems kinda out of scope. Richard, can you update the patch
description and comment so that they clearly state that this is a debug aid
to help spotting user errors?

Thanks.

-- 
tejun

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