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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Sun, 26 Apr 2020 17:00:27 -0700
Message-ID: <CAJuCfpECDryYL7ia+JH8HZtWRWHtLFFg2ZwtAB4nj=mXDDPENg@mail.gmail.com>
Subject: Re: lockdep warning about possible circular dependency in PSI
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>, will@kernel.org,
        Johannes Weiner <hannes@cmpxchg.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@android.com>
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On Fri, Apr 24, 2020 at 1:54 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Fri, Apr 24, 2020 at 10:52 AM Suren Baghdasaryan <surenb@google.com> w=
rote:
> >
> > On Fri, Apr 24, 2020 at 10:40 AM Peter Zijlstra <peterz@infradead.org> =
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@google.c=
om> wrote:
> > > > >
> > > > > I received a report about possible circular locking dependency wa=
rning
> > > > > generated from PSI polling code. I think we are protected from th=
is
> > > > > scenario by poll_scheduled atomic but wanted to double-check and =
I=E2=80=99m
> > > > > looking for an advice on how to annotate this case to fix the loc=
kdep
> > > > > warning. I copied the detailed information at the end of this ema=
il
> > > > > but the short story is this:
> > > > >
> > > > > "WARNING: possible circular locking dependency detected" is gener=
ated
> > > > > 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!
> >
>
> I tested on my simple repro and the patch fixes the lockdep splat.
>
> You can add
> Tested-by: Shakeel Butt <shakeelb@google.com>
>

Thanks Shakeel! Will do.

> > > ---
> > > 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(st=
ruct 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 =3D 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 =3D false;
> > > +       DEFINE_WAKE_Q(wake_q);
> > >         unsigned long flags;
> > > +       bool ret =3D 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 =3D 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_l=
ist *t)
> > >         struct kthread_delayed_work *dwork =3D from_timer(dwork, t, t=
imer);
> > >         struct kthread_work *work =3D &dwork->work;
> > >         struct kthread_worker *worker =3D 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 *work=
er,
> > >                                          struct kthread_delayed_work =
*dwork,
> > > -                                        unsigned long delay)
> > > +                                        unsigned long delay,
> > > +                                        struct wake_q_head *wake_q)
> > >  {
> > >         struct timer_list *timer =3D &dwork->timer;
> > >         struct kthread_work *work =3D &dwork->work;
> > > @@ -895,7 +904,7 @@ static void __kthread_queue_delayed_work(struct k=
thread_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 =3D &dwork->work;
> > > +       DEFINE_WAKE_Q(wake_q);
> > >         unsigned long flags;
> > >         bool ret =3D 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, &w=
ake_q);
> > >                 ret =3D 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 =3D false;
> > >
> > > @@ -979,15 +993,17 @@ void kthread_flush_work(struct kthread_work *wo=
rk)
> > >         WARN_ON_ONCE(work->worker !=3D worker);
> > >
> > >         if (!list_empty(&work->node))
> > > -               kthread_insert_work(worker, &fwork.work, work->node.n=
ext);
> > > +               kthread_insert_work(worker, &fwork.work, work->node.n=
ext, &wake_q);
> > >         else if (worker->current_work =3D=3D work)
> > >                 kthread_insert_work(worker, &fwork.work,
> > > -                                   worker->work_list.next);
> > > +                                   worker->work_list.next, &wake_q);
> > >         else
> > >                 noop =3D 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_wo=
rker *worker,
> > >                               unsigned long delay)
> > >  {
> > >         struct kthread_work *work =3D &dwork->work;
> > > +       DEFINE_WAKE_Q(wake_q);
> > >         unsigned long flags;
> > >         int ret =3D false;
> > >
> > > @@ -1083,9 +1100,12 @@ bool kthread_mod_delayed_work(struct kthread_w=
orker *worker,
> > >
> > >         ret =3D __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);
> > >
> > > --
> > > To unsubscribe from this group and stop receiving emails from it, sen=
d an email to kernel-team+unsubscribe@android.com.
> > >