Re: work item still be scheduled to execute after destroy_workqueue?

[richard clark]

On Thu, Dec 8, 2022 at 3:46 PM Lai Jiangshan <jiangshanlai@xxxxxxxxx> wrote:
>
> On Thu, Dec 8, 2022 at 10:44 AM richard clark
> <richard.xnu.clark@xxxxxxxxx> wrote:
> >
> > On Wed, Dec 7, 2022 at 10:38 AM Lai Jiangshan <jiangshanlai@xxxxxxxxx> wrote:
> > >
> > > On Tue, Dec 6, 2022 at 5:20 PM richard clark
> > > <richard.xnu.clark@xxxxxxxxx> wrote:
> > > >
> > > > On Tue, Dec 6, 2022 at 2:23 PM Lai Jiangshan <jiangshanlai@xxxxxxxxx> wrote:
> > > > >
> > > > > On Tue, Dec 6, 2022 at 12:35 PM richard clark
> > > > > <richard.xnu.clark@xxxxxxxxx> wrote:
> > > > >
> > > > > > >
> > > > > > A WARN is definitely reasonable and has its benefits. Can I try to
> > > > > > submit the patch and you're nice to review as maintainer?
> > > > > >
> > > > > > Thanks,
> > > > > > Richard
> > > > > > >
> > > > >
> > > > > Sure, go ahead.
> > > > >
> > > > > What's in my mind is that the following code is wrapped in a new function:
> > > > >
> > > > >         mutex_lock(&wq->mutex);
> > > > >         if (!wq->nr_drainers++)
> > > > >                 wq->flags |= __WQ_DRAINING;
> > > > >         mutex_unlock(&wq->mutex);
> > > > >
> > > > >
> > > > > and the new function replaces the open code drain_workqueue() and
> > > > > is also called in destroy_workqueue() (before calling drain_workqueue()).
> > > > >
> > > > Except that, do we need to defer the __WQ_DRAINING clean to the
> > > > rcu_call, thus we still have a close-loop of the drainer's count, like
> > > > this?
> > >
> > > No, I don't think we need it. The wq is totally freed in rcu_free_wq.
> > >
> > > Or we can just introduce __WQ_DESTROYING.
> > >
> > > It seems using __WQ_DESTROYING is better.
> >
> > The wq->flags will be unreliable after kfree(wq), for example, in my
> > machine, the wq->flags can be 0x7ec1e1a3, 0x37cff1a3 or 0x7fa23da3 ...
> > after wq be kfreed, consequently the result of queueing a new work
> > item to a kfreed wq is undetermined, sometimes it's ok because the
> > queue_work will return directly(e.g, the wq->flags = 0x7ec1e1a3, a
> > fake __WQ_DRAINING state), sometimes it will trigger a kernel NULL
> > pointer dereference BUG when the wq->flags = 0x7fa23da3(fake
> > !__WQ_DRAINING state).
>
> The whole wq is unreliable after destroy_workqueue().
>
> All we need is just adding something to help identify any
> wrong usage while the wq is in RCU grace period.
>
OK, understood!
> >
> > IMO, given the above condition,  we can handle this in 2 phases:
> > before the rcu_call and after.
> > a. before rcu_call. Using __WQ_DESTROYING to allow the chained work
> > queued in or not in destroy_workqueue(...) level, __WQ_DRAINING is
> > used to make the drain_workqueue(...) still can be standalone. The
> > code snippet like this:
> > destroy_workqueue(...)
> > {
> >         mutex_lock(&wq->mutex);
> >         wq->flags |= __WQ_DESTROYING;
> >         mutex_lock(&wq->mutex);
>
> Ok, put it before calling drain_workqueue()
>
> >         ...
> > }
> >
> > __queue_work(...)
> > {
> >           if (unlikely((wq->flags & __WQ_DESTROYING) || (wq->flags &
> > __WQ_DRAINING)) &&
> >                    WARN_ON_ONCE(!is_chained_work(wq)))
>
> Ok, combine __WQ_DESTROYING and __WQ_DRAINING together as:
>            if (unlikely((wq->flags & (__WQ_DESTROYING | __WQ_DRAINING)) &&
>
>
> >          return;
> > }
> >
> > b. after rcu_call. What in my mind is:
> > rcu_free_wq(struct rcu_head *rcu)
> > {
> >           ...
> >           kfree(wq);
> >           wq = NULL;
>
> It is useless code.
>
> > }
> >
> > __queue_work(...)
> > {
> >         if (!wq)
> >                 return;
>
> It is useless code.

OK, will remove the above codes in the patch...

>
> >         ...
> > }
> >
> > Any comments?
> >
> > >
> > > >
> > > > --- a/kernel/workqueue.c
> > > > +++ b/kernel/workqueue.c
> > > >
> > > > @@ -3528,6 +3526,9 @@ static void rcu_free_wq(struct rcu_head *rcu)
> > > >
> > > >         else
> > > >                 free_workqueue_attrs(wq->unbound_attrs);
> > > >
> > > > +       if (!--wq->nr_drainers)
> > > > +               wq->flags &= ~__WQ_DRAINING;
> > > > +
> > > >         kfree(wq);
> > > >
> > > > >
> > > > > __WQ_DRAINING will cause the needed WARN on illegally queuing items on
> > > > > destroyed workqueue.
> > > >
> > > > I will re-test it if there are no concerns about the above fix...
> > > >
> > > > >
> > > > > Thanks
> > > > > Lai