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Subject[PATCH 1/2] sched: Fix balance_callback()
The intent of balance_callback() has always been to delay executing
balancing operations until the end of the current rq->lock section.
This is because balance operations must often drop rq->lock, and that
isn't safe in general.

However, as noted by Scott, there were a few holes in that scheme;
balance_callback() was called after rq->lock was dropped, which means
another CPU can interleave and touch the callback list.

Rework code to call the balance callbacks before dropping rq->lock
where possible, and otherwise splice the balance list onto a local
stack.

This guarantees that the balance list must be empty when we take
rq->lock. IOW, we'll only ever run our own balance callbacks.

Reported-by: Scott Wood <swood@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/sched/core.c | 119 ++++++++++++++++++++++++++++++++-------------------
kernel/sched/sched.h | 2
2 files changed, 77 insertions(+), 44 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3489,6 +3489,69 @@ static inline void finish_task(struct ta
#endif
}

+#ifdef CONFIG_SMP
+
+static void do_balance_callbacks(struct rq *rq, struct callback_head *head)
+{
+ void (*func)(struct rq *rq);
+ struct callback_head *next;
+
+ lockdep_assert_held(&rq->lock);
+
+ while (head) {
+ func = (void (*)(struct rq *))head->func;
+ next = head->next;
+ head->next = NULL;
+ head = next;
+
+ func(rq);
+ }
+}
+
+static inline struct callback_head *splice_balance_callbacks(struct rq *rq)
+{
+ struct callback_head *head = rq->balance_callback;
+
+ lockdep_assert_held(&rq->lock);
+ if (head)
+ rq->balance_callback = NULL;
+
+ return head;
+}
+
+static void __balance_callbacks(struct rq *rq)
+{
+ do_balance_callbacks(rq, splice_balance_callbacks(rq));
+}
+
+static inline void balance_callbacks(struct rq *rq, struct callback_head *head)
+{
+ unsigned long flags;
+
+ if (unlikely(head)) {
+ raw_spin_lock_irqsave(&rq->lock, flags);
+ do_balance_callbacks(rq, head);
+ raw_spin_unlock_irqrestore(&rq->lock, flags);
+ }
+}
+
+#else
+
+static inline void __balance_callbacks(struct rq *rq)
+{
+}
+
+static inline struct callback_head *splice_balance_callbacks(struct rq *rq)
+{
+ return NULL;
+}
+
+static inline void balance_callbacks(struct rq *rq, struct callback_head *head)
+{
+}
+
+#endif
+
static inline void
prepare_lock_switch(struct rq *rq, struct task_struct *next, struct rq_flags *rf)
{
@@ -3514,6 +3577,7 @@ static inline void finish_lock_switch(st
* prev into current:
*/
spin_acquire(&rq->lock.dep_map, 0, 0, _THIS_IP_);
+ __balance_callbacks(rq);
raw_spin_unlock_irq(&rq->lock);
}

@@ -3655,43 +3719,6 @@ static struct rq *finish_task_switch(str
return rq;
}

-#ifdef CONFIG_SMP
-
-/* rq->lock is NOT held, but preemption is disabled */
-static void __balance_callback(struct rq *rq)
-{
- struct callback_head *head, *next;
- void (*func)(struct rq *rq);
- unsigned long flags;
-
- raw_spin_lock_irqsave(&rq->lock, flags);
- head = rq->balance_callback;
- rq->balance_callback = NULL;
- while (head) {
- func = (void (*)(struct rq *))head->func;
- next = head->next;
- head->next = NULL;
- head = next;
-
- func(rq);
- }
- raw_spin_unlock_irqrestore(&rq->lock, flags);
-}
-
-static inline void balance_callback(struct rq *rq)
-{
- if (unlikely(rq->balance_callback))
- __balance_callback(rq);
-}
-
-#else
-
-static inline void balance_callback(struct rq *rq)
-{
-}
-
-#endif
-
/**
* schedule_tail - first thing a freshly forked thread must call.
* @prev: the thread we just switched away from.
@@ -3711,7 +3738,6 @@ asmlinkage __visible void schedule_tail(
*/

rq = finish_task_switch(prev);
- balance_callback(rq);
preempt_enable();

if (current->set_child_tid)
@@ -4527,10 +4553,11 @@ static void __sched notrace __schedule(b
rq = context_switch(rq, prev, next, &rf);
} else {
rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
- rq_unlock_irq(rq, &rf);
- }

- balance_callback(rq);
+ rq_unpin_lock(rq, &rf);
+ __balance_callbacks(rq);
+ raw_spin_unlock_irq(&rq->lock);
+ }
}

void __noreturn do_task_dead(void)
@@ -4941,9 +4968,11 @@ void rt_mutex_setprio(struct task_struct
out_unlock:
/* Avoid rq from going away on us: */
preempt_disable();
- __task_rq_unlock(rq, &rf);

- balance_callback(rq);
+ rq_unpin_lock(rq, &rf);
+ __balance_callbacks(rq);
+ raw_spin_unlock(&rq->lock);
+
preempt_enable();
}
#else
@@ -5217,6 +5246,7 @@ static int __sched_setscheduler(struct t
int retval, oldprio, oldpolicy = -1, queued, running;
int new_effective_prio, policy = attr->sched_policy;
const struct sched_class *prev_class;
+ struct callback_head *head;
struct rq_flags rf;
int reset_on_fork;
int queue_flags = DEQUEUE_SAVE | DEQUEUE_MOVE | DEQUEUE_NOCLOCK;
@@ -5455,6 +5485,7 @@ static int __sched_setscheduler(struct t

/* Avoid rq from going away on us: */
preempt_disable();
+ head = splice_balance_callbacks(rq);
task_rq_unlock(rq, p, &rf);

if (pi) {
@@ -5463,7 +5494,7 @@ static int __sched_setscheduler(struct t
}

/* Run balance callbacks after we've adjusted the PI chain: */
- balance_callback(rq);
+ balance_callbacks(rq, head);
preempt_enable();

return 0;
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1220,6 +1220,8 @@ static inline void rq_pin_lock(struct rq
#ifdef CONFIG_SCHED_DEBUG
rq->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
rf->clock_update_flags = 0;
+
+ SCHED_WARN_ON(rq->balance_callback);
#endif
}


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