Re: lockdep warning about possible circular dependency in PSI

[Suren Baghdasaryan]

On Fri, Apr 24, 2020 at 10:40 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Fri, Apr 24, 2020 at 09:34:42AM -0700, Suren Baghdasaryan wrote:
> > Sorry to bother you again folks. Any suggestions on how to silence
> > this lockdep warning which I believe to be a false positive?
> >
> > On Wed, Apr 15, 2020 at 4:01 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > >
> > > I received a report about possible circular locking dependency warning
> > > generated from PSI polling code. I think we are protected from this
> > > scenario by poll_scheduled atomic but wanted to double-check and Iâm
> > > looking for an advice on how to annotate this case to fix the lockdep
> > > warning. I copied the detailed information at the end of this email
> > > but the short story is this:
> > >
> > > "WARNING: possible circular locking dependency detected" is generated
> > > with CONFIG_PSI and CONFIG_LOCKDEP enabled. The dependency chain it
> > > describes is:
> > >
> > > #0
> > > kthread_delayed_work_timer_fn()
> > >  |
> > > worker->lock
> > >  |
> > > try_to_wake_up()
> > >  |
> > > p->pi_lock
> > >
> > > #1
> > > sched_fork()
> > >  |
> > > p->pi_lock
> > >  |
> > > task_fork_fair()
> > >  |
> > > rq->lock
> > >
> > > #2
> > > psi_memstall_enter
> > >  |
> > > rq->lock
> > >  |
> > > kthread_queue_delayed_work()
> > >  |
> > > worker->lock
>
> Irrespective of it actually being a deadlock or not, it is fairly
> fragile. Ideally we'd fix #2, we really should minimize the number of
> locks nested under rq->lock.
>
> That said, here's the easy fix, which breaks #0.
>

Thanks for the suggestion, Peter. Let me digest this and will post a
patch with your Suggested-by.
Cheers!

> ---
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index bfbfa481be3a..b443bba7dd21 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -806,14 +806,15 @@ static void kthread_insert_work_sanity_check(struct kthread_worker *worker,
>  /* insert @work before @pos in @worker */
>  static void kthread_insert_work(struct kthread_worker *worker,
>                                 struct kthread_work *work,
> -                               struct list_head *pos)
> +                               struct list_head *pos,
> +                               struct wake_q_head *wake_q)
>  {
>         kthread_insert_work_sanity_check(worker, work);
>
>         list_add_tail(&work->node, pos);
>         work->worker = worker;
>         if (!worker->current_work && likely(worker->task))
> -               wake_up_process(worker->task);
> +               wake_q_add(wake_q, worker->task);
>  }
>
>  /**
> @@ -831,15 +832,19 @@ static void kthread_insert_work(struct kthread_worker *worker,
>  bool kthread_queue_work(struct kthread_worker *worker,
>                         struct kthread_work *work)
>  {
> -       bool ret = false;
> +       DEFINE_WAKE_Q(wake_q);
>         unsigned long flags;
> +       bool ret = false;
>
>         raw_spin_lock_irqsave(&worker->lock, flags);
>         if (!queuing_blocked(worker, work)) {
> -               kthread_insert_work(worker, work, &worker->work_list);
> +               kthread_insert_work(worker, work, &worker->work_list, &wake_q);
>                 ret = true;
>         }
>         raw_spin_unlock_irqrestore(&worker->lock, flags);
> +
> +       wake_up_q(&wake_q);
> +
>         return ret;
>  }
>  EXPORT_SYMBOL_GPL(kthread_queue_work);
> @@ -857,6 +862,7 @@ void kthread_delayed_work_timer_fn(struct timer_list *t)
>         struct kthread_delayed_work *dwork = from_timer(dwork, t, timer);
>         struct kthread_work *work = &dwork->work;
>         struct kthread_worker *worker = work->worker;
> +       DEFINE_WAKE_Q(wake_q);
>         unsigned long flags;
>
>         /*
> @@ -873,15 +879,18 @@ void kthread_delayed_work_timer_fn(struct timer_list *t)
>         /* Move the work from worker->delayed_work_list. */
>         WARN_ON_ONCE(list_empty(&work->node));
>         list_del_init(&work->node);
> -       kthread_insert_work(worker, work, &worker->work_list);
> +       kthread_insert_work(worker, work, &worker->work_list, &wake_q);
>
>         raw_spin_unlock_irqrestore(&worker->lock, flags);
> +
> +       wake_up_q(&wake_q);
>  }
>  EXPORT_SYMBOL(kthread_delayed_work_timer_fn);
>
>  static void __kthread_queue_delayed_work(struct kthread_worker *worker,
>                                          struct kthread_delayed_work *dwork,
> -                                        unsigned long delay)
> +                                        unsigned long delay,
> +                                        struct wake_q_head *wake_q)
>  {
>         struct timer_list *timer = &dwork->timer;
>         struct kthread_work *work = &dwork->work;
> @@ -895,7 +904,7 @@ static void __kthread_queue_delayed_work(struct kthread_worker *worker,
>          * on that there's no such delay when @delay is 0.
>          */
>         if (!delay) {
> -               kthread_insert_work(worker, work, &worker->work_list);
> +               kthread_insert_work(worker, work, &worker->work_list, wake_q);
>                 return;
>         }
>
> @@ -928,17 +937,21 @@ bool kthread_queue_delayed_work(struct kthread_worker *worker,
>                                 unsigned long delay)
>  {
>         struct kthread_work *work = &dwork->work;
> +       DEFINE_WAKE_Q(wake_q);
>         unsigned long flags;
>         bool ret = false;
>
>         raw_spin_lock_irqsave(&worker->lock, flags);
>
>         if (!queuing_blocked(worker, work)) {
> -               __kthread_queue_delayed_work(worker, dwork, delay);
> +               __kthread_queue_delayed_work(worker, dwork, delay, &wake_q);
>                 ret = true;
>         }
>
>         raw_spin_unlock_irqrestore(&worker->lock, flags);
> +
> +       wake_up_q(&wake_q);
> +
>         return ret;
>  }
>  EXPORT_SYMBOL_GPL(kthread_queue_delayed_work);
> @@ -967,6 +980,7 @@ void kthread_flush_work(struct kthread_work *work)
>                 KTHREAD_WORK_INIT(fwork.work, kthread_flush_work_fn),
>                 COMPLETION_INITIALIZER_ONSTACK(fwork.done),
>         };
> +       DEFINE_WAKE_Q(wake_q);
>         struct kthread_worker *worker;
>         bool noop = false;
>
> @@ -979,15 +993,17 @@ void kthread_flush_work(struct kthread_work *work)
>         WARN_ON_ONCE(work->worker != worker);
>
>         if (!list_empty(&work->node))
> -               kthread_insert_work(worker, &fwork.work, work->node.next);
> +               kthread_insert_work(worker, &fwork.work, work->node.next, &wake_q);
>         else if (worker->current_work == work)
>                 kthread_insert_work(worker, &fwork.work,
> -                                   worker->work_list.next);
> +                                   worker->work_list.next, &wake_q);
>         else
>                 noop = true;
>
>         raw_spin_unlock_irq(&worker->lock);
>
> +       wake_up_q(&wake_q);
> +
>         if (!noop)
>                 wait_for_completion(&fwork.done);
>  }
> @@ -1065,6 +1081,7 @@ bool kthread_mod_delayed_work(struct kthread_worker *worker,
>                               unsigned long delay)
>  {
>         struct kthread_work *work = &dwork->work;
> +       DEFINE_WAKE_Q(wake_q);
>         unsigned long flags;
>         int ret = false;
>
> @@ -1083,9 +1100,12 @@ bool kthread_mod_delayed_work(struct kthread_worker *worker,
>
>         ret = __kthread_cancel_work(work, true, &flags);
>  fast_queue:
> -       __kthread_queue_delayed_work(worker, dwork, delay);
> +       __kthread_queue_delayed_work(worker, dwork, delay, &wake_q);
>  out:
>         raw_spin_unlock_irqrestore(&worker->lock, flags);
> +
> +       wake_up_q(&wake_q);
> +
>         return ret;
>  }
>  EXPORT_SYMBOL_GPL(kthread_mod_delayed_work);
>
> --
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