Re: lockdep warning about possible circular dependency in PSI
From: Shakeel Butt
Date: Fri Apr 24 2020 - 16:54:15 EST
On Fri, Apr 24, 2020 at 10:52 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> 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!
>
I tested on my simple repro and the patch fixes the lockdep splat.
You can add
Tested-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > ---
> > 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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> >