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.

Peter Zijlstra committed 6 years ago

Commit 565790d1 parent a8b62fd2 files · +78 −44Browse files

Unified · Split

2 files

kernel/sched/core.c +75 −44
kernel/sched/sched.h +3 −0

kernel/sched/core.c+75−44

 #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)
 {
 	 * prev into current:
 	 */
 	spin_acquire(&rq->lock.dep_map, 0, 0, _THIS_IP_);
+	__balance_callbacks(rq);
 	raw_spin_unlock_irq(&rq->lock);
 }
 	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.
 	 */
 	rq = finish_task_switch(prev);
-	balance_callback(rq);
 	preempt_enable();
 	if (current->set_child_tid)
 		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)
 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
 	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;
 	/* Avoid rq from going away on us: */
 	preempt_disable();
+	head = splice_balance_callbacks(rq);
 	task_rq_unlock(rq, p, &rf);
 	if (pi) {
 	}
 	/* Run balance callbacks after we've adjusted the PI chain: */
-	balance_callback(rq);
+	balance_callbacks(rq, head);
 	preempt_enable();
 	return 0;

kernel/sched/sched.h+3−0

 	rq->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
 	rf->clock_update_flags = 0;
 #endif
+#ifdef CONFIG_SMP
+	SCHED_WARN_ON(rq->balance_callback);
+#endif
 }
 static inline void rq_unpin_lock(struct rq *rq, struct rq_flags *rf)

sched: Fix balance_callback()

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