Re: lockdep warning about possible circular dependency in PSI

From: Peter Zijlstra
Date: Fri Apr 24 2020 - 13:40:40 EST

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.

---
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);