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From:   richard clark <richard.xnu.clark@gmail.com>
Date:   Tue, 13 Dec 2022 09:09:48 +0800
Message-ID: <CAJNi4rOpm=6m6Lm65QP-qLvbR19RiyJbvgJNhKfZJWLH7WHR5A@mail.gmail.com>
Subject: Re: [PATCH] workqueue: Prevent a new work item from queueing into a
 destruction wq
To:     Tejun Heo <tj@kernel.org>
Cc:     Lai Jiangshan <jiangshanlai@gmail.com>,
        linux-kernel@vger.kernel.org
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On Tue, Dec 13, 2022 at 6:27 AM Tejun Heo <tj@kernel.org> wrote:
>
> 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@kernel.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?

Sure, will update soon

Thanks

>
> Thanks.
>
> --
> tejun