linux-kernel - Re: work item still be scheduled to execute after destroy

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Message-ID: <CAJhGHyC=-bD8L5KSYLp3_tN0WdoCYgtpwNaM_=oLb2c=Gsu+6A@mail.gmail.com>
Date:   Wed, 7 Dec 2022 10:37:49 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     richard clark <richard.xnu.clark@...il.com>
Cc:     tj@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: work item still be scheduled to execute after destroy_workqueue?

On Tue, Dec 6, 2022 at 5:20 PM richard clark
<richard.xnu.clark@...il.com> wrote:
>
> On Tue, Dec 6, 2022 at 2:23 PM Lai Jiangshan <jiangshanlai@...il.com> wrote:
> >
> > On Tue, Dec 6, 2022 at 12:35 PM richard clark
> > <richard.xnu.clark@...il.com> 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.

>
> --- 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