Re: [PATCH] workqueue: Prevent a new work item from queueing into a destruction wq

[richard clark]

On Mon, Dec 12, 2022 at 2:48 PM Lai Jiangshan <jiangshanlai@xxxxxxxxx> wrote:
>
> On Mon, Dec 12, 2022 at 2:23 PM Tejun Heo <tj@xxxxxxxxxx> 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.

Agree...

>
> 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.

This case will trigger a noticeable kernel BUG

>
> 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.
>

    destroy_workqueue(wq0);
    schedule_timeout_interruptible(msecs_to_jiffies(1000));
    queue_work_on(1, wq0, &w0);

Sleep 1s to guarantee the RCU grace period completes, then the same
result with non-RCU free

Thanks

>
>
> Thanks
> Lai