[PATCH 1/1] kthread: break dependency between worker->lock and task_struct->pi_lock

From: Suren Baghdasaryan
Date: Mon Apr 27 2020 - 14:44:07 EST

A number of kthread-related functions indirectly take task_struct->pi_lock
while holding worker->lock in the call chain like this:
spin_lock(&worker->lock)
kthread_insert_work
wake_up_process
try_to_wake_up
raw_spin_lock_irqsave(&p->pi_lock, flags)

This lock dependency exists whenever kthread_insert_work is called either
directly or indirectly via __kthread_queue_delayed_work in the following
functions:
kthread_queue_work
kthread_delayed_work_timer_fn
kthread_queue_delayed_work
kthread_flush_work
kthread_mod_delayed_work

This creates possibilities for circular dependencies like the one reported
at: https://lkml.org/lkml/2020/4/24/954
Break this lock dependency by moving task wakeup after worker->lock has
been released.

Reported-by: Ke Wang <ke.wang@xxxxxxxxxx>
Reported-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Tested-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
---
kernel/kthread.c | 44 +++++++++++++++++++++++++++++++++-----------
1 file changed, 33 insertions(+), 11 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index bfbfa481be3a..7a93654072bb 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -9,6 +9,7 @@
#include <uapi/linux/sched/types.h>
#include <linux/sched.h>
#include <linux/sched/task.h>
+#include <linux/sched/wake_q.h>
#include <linux/kthread.h>
#include <linux/completion.h>
#include <linux/err.h>
@@ -806,14 +807,15 @@ 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 +833,19 @@ 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 +863,7 @@ 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 +880,18 @@ 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 +905,7 @@ 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 +938,21 @@ 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 +981,7 @@ 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 +994,18 @@ 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 +1083,7 @@ 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 +1102,12 @@ 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);
--
2.26.2.303.gf8c07b1a785-goog