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From:   richard clark <richard.xnu.clark@gmail.com>
Date:   Mon, 12 Dec 2022 17:17:26 +0800
Message-ID: <CAJNi4rNzSOyRt01G6=EYH-ojVHPaOLDJVnLbfeehXbcTxBEa=g@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:     jiangshanlai@gmail.com, linux-kernel@vger.kernel.org
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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

IMO, it's not exactly the use-after-free since no free action before
the end of RCU grace period, if it really is then the code will
trigger a kernel BUG:

    BUG: kernel NULL pointer dereference, address: 0000000000000000
    #PF: supervisor read access in kernel mode
    #PF: error_code(0x0000) - not-present page
    ...

Which can be easily observed for both non-RCU frees and RCU frees.

Thanks

> non-RCU frees or if there happens to be an RCU grace period inbetween. I'm
> not sure what's being protected here.
>
> Thanks.
>
> --
> tejun