tasklet-redesign-make-it-saner-and-make-it-easier-to.patch

From 20f932344f39d498fa68c212eb069c2ad5fccf3e Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@elte.hu>
Date: Fri, 3 Jul 2009 08:29:56 -0500
Subject: [PATCH] tasklet: redesign: make it saner and make it easier to thread.

commit 2b3582fb698056add3719fde4e7bba0d79bf7b5c in tip.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

----
 include/linux/interrupt.h |   33 ++++----
 kernel/softirq.c          |  184 ++++++++++++++++++++++++++++++++--------------
 2 files changed, 149 insertions(+), 68 deletions(-)

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index 563f51e..86cf03e 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -424,8 +424,9 @@ extern void __send_remote_softirq(struct call_single_data *cp, int cpu,
      to be executed on some cpu at least once after this.
    * If the tasklet is already scheduled, but its excecution is still not
      started, it will be executed only once.
-   * If this tasklet is already running on another CPU (or schedule is called
-     from tasklet itself), it is rescheduled for later.
+   * If this tasklet is already running on another CPU, it is rescheduled
+     for later.
+   * Schedule must not be called from the tasklet itself (a lockup occurs)
    * Tasklet is strictly serialized wrt itself, but not
      wrt another tasklets. If client needs some intertask synchronization,
      he makes it with spinlocks.
@@ -450,15 +451,25 @@ struct tasklet_struct name = { NULL, 0, ATOMIC_INIT(1), func, data }
 enum
 {
 	TASKLET_STATE_SCHED,	/* Tasklet is scheduled for execution */
-	TASKLET_STATE_RUN	/* Tasklet is running (SMP only) */
+	TASKLET_STATE_RUN,	/* Tasklet is running (SMP only) */
+	TASKLET_STATE_PENDING	/* Tasklet is pending */
 };
 
-#ifdef CONFIG_SMP
+#define TASKLET_STATEF_SCHED	(1 << TASKLET_STATE_SCHED)
+#define TASKLET_STATEF_RUN	(1 << TASKLET_STATE_RUN)
+#define TASKLET_STATEF_PENDING	(1 << TASKLET_STATE_PENDING)
+
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT)
 static inline int tasklet_trylock(struct tasklet_struct *t)
 {
 	return !test_and_set_bit(TASKLET_STATE_RUN, &(t)->state);
 }
 
+static inline int tasklet_tryunlock(struct tasklet_struct *t)
+{
+	return cmpxchg(&t->state, TASKLET_STATEF_RUN, 0) == TASKLET_STATEF_RUN;
+}
+
 static inline void tasklet_unlock(struct tasklet_struct *t)
 {
 	smp_mb__before_clear_bit(); 
@@ -471,6 +482,7 @@ static inline void tasklet_unlock_wait(struct tasklet_struct *t)
 }
 #else
 #define tasklet_trylock(t) 1
+#define tasklet_tryunlock(t)	1
 #define tasklet_unlock_wait(t) do { } while (0)
 #define tasklet_unlock(t) do { } while (0)
 #endif
@@ -519,17 +531,8 @@ static inline void tasklet_disable(struct tasklet_struct *t)
 	smp_mb();
 }
 
-static inline void tasklet_enable(struct tasklet_struct *t)
-{
-	smp_mb__before_atomic_dec();
-	atomic_dec(&t->count);
-}
-
-static inline void tasklet_hi_enable(struct tasklet_struct *t)
-{
-	smp_mb__before_atomic_dec();
-	atomic_dec(&t->count);
-}
+extern  void tasklet_enable(struct tasklet_struct *t);
+extern  void tasklet_hi_enable(struct tasklet_struct *t);
 
 extern void tasklet_kill(struct tasklet_struct *t);
 extern void tasklet_kill_immediate(struct tasklet_struct *t, unsigned int cpu);
diff --git a/kernel/softirq.c b/kernel/softirq.c
index eda9c66..1be5ff4 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -398,15 +398,45 @@ struct tasklet_head
 static DEFINE_PER_CPU(struct tasklet_head, tasklet_vec);
 static DEFINE_PER_CPU(struct tasklet_head, tasklet_hi_vec);
 
+static void inline
+__tasklet_common_schedule(struct tasklet_struct *t, struct tasklet_head *head, unsigned int nr)
+{
+	if (tasklet_trylock(t)) {
+again:
+		/* We may have been preempted before tasklet_trylock
+		 * and __tasklet_action may have already run.
+		 * So double check the sched bit while the takslet
+		 * is locked before adding it to the list.
+		 */
+		if (test_bit(TASKLET_STATE_SCHED, &t->state)) {
+			t->next = NULL;
+			*head->tail = t;
+			head->tail = &(t->next);
+			raise_softirq_irqoff(nr);
+			tasklet_unlock(t);
+		} else {
+			/* This is subtle. If we hit the corner case above
+			 * It is possible that we get preempted right here,
+			 * and another task has successfully called
+			 * tasklet_schedule(), then this function, and
+			 * failed on the trylock. Thus we must be sure
+			 * before releasing the tasklet lock, that the
+			 * SCHED_BIT is clear. Otherwise the tasklet
+			 * may get its SCHED_BIT set, but not added to the
+			 * list
+			 */
+			if (!tasklet_tryunlock(t))
+				goto again;
+		}
+	}
+}
+
 void __tasklet_schedule(struct tasklet_struct *t)
 {
 	unsigned long flags;
 
 	local_irq_save(flags);
-	t->next = NULL;
-	*__get_cpu_var(tasklet_vec).tail = t;
-	__get_cpu_var(tasklet_vec).tail = &(t->next);
-	raise_softirq_irqoff(TASKLET_SOFTIRQ);
+	__tasklet_common_schedule(t, &__get_cpu_var(tasklet_vec), TASKLET_SOFTIRQ);
 	local_irq_restore(flags);
 }
 
@@ -417,10 +447,7 @@ void __tasklet_hi_schedule(struct tasklet_struct *t)
 	unsigned long flags;
 
 	local_irq_save(flags);
-	t->next = NULL;
-	*__get_cpu_var(tasklet_hi_vec).tail = t;
-	__get_cpu_var(tasklet_hi_vec).tail = &(t->next);
-	raise_softirq_irqoff(HI_SOFTIRQ);
+	__tasklet_common_schedule(t, &__get_cpu_var(tasklet_vec), HI_SOFTIRQ);
 	local_irq_restore(flags);
 }
 
@@ -437,74 +464,125 @@ void __tasklet_hi_schedule_first(struct tasklet_struct *t)
 
 EXPORT_SYMBOL(__tasklet_hi_schedule_first);
 
-static void tasklet_action(struct softirq_action *a)
+void  tasklet_enable(struct tasklet_struct *t)
 {
-	struct tasklet_struct *list;
+	if (!atomic_dec_and_test(&t->count))
+		return;
+	if (test_and_clear_bit(TASKLET_STATE_PENDING, &t->state))
+		tasklet_schedule(t);
+}
 
-	local_irq_disable();
-	list = __get_cpu_var(tasklet_vec).head;
-	__get_cpu_var(tasklet_vec).head = NULL;
-	__get_cpu_var(tasklet_vec).tail = &__get_cpu_var(tasklet_vec).head;
-	local_irq_enable();
+EXPORT_SYMBOL(tasklet_enable);
+
+void  tasklet_hi_enable(struct tasklet_struct *t)
+{
+	if (!atomic_dec_and_test(&t->count))
+		return;
+	if (test_and_clear_bit(TASKLET_STATE_PENDING, &t->state))
+		tasklet_hi_schedule(t);
+}
+
+EXPORT_SYMBOL(tasklet_hi_enable);
+
+static void
+__tasklet_action(struct softirq_action *a, struct tasklet_struct *list)
+{
+	int loops = 1000000;
 
 	while (list) {
 		struct tasklet_struct *t = list;
 
 		list = list->next;
 
-		if (tasklet_trylock(t)) {
-			if (!atomic_read(&t->count)) {
-				if (!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
-					BUG();
-				t->func(t->data);
-				tasklet_unlock(t);
-				continue;
-			}
-			tasklet_unlock(t);
+		/*
+		 * Should always succeed - after a tasklist got on the
+		 * list (after getting the SCHED bit set from 0 to 1),
+		 * nothing but the tasklet softirq it got queued to can
+		 * lock it:
+		 */
+		if (!tasklet_trylock(t)) {
+			WARN_ON(1);
+			continue;
 		}
 
-		local_irq_disable();
 		t->next = NULL;
-		*__get_cpu_var(tasklet_vec).tail = t;
-		__get_cpu_var(tasklet_vec).tail = &(t->next);
-		__do_raise_softirq_irqoff(TASKLET_SOFTIRQ);
-		local_irq_enable();
+
+		/*
+		 * If we cannot handle the tasklet because it's disabled,
+		 * mark it as pending. tasklet_enable() will later
+		 * re-schedule the tasklet.
+		 */
+		if (unlikely(atomic_read(&t->count))) {
+out_disabled:
+			/* implicit unlock: */
+			wmb();
+			t->state = TASKLET_STATEF_PENDING;
+			continue;
+		}
+
+		/*
+		 * After this point on the tasklet might be rescheduled
+		 * on another CPU, but it can only be added to another
+		 * CPU's tasklet list if we unlock the tasklet (which we
+		 * dont do yet).
+		 */
+		if (!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
+			WARN_ON(1);
+
+again:
+		t->func(t->data);
+
+		/*
+		 * Try to unlock the tasklet. We must use cmpxchg, because
+		 * another CPU might have scheduled or disabled the tasklet.
+		 * We only allow the STATE_RUN -> 0 transition here.
+		 */
+		while (!tasklet_tryunlock(t)) {
+			/*
+			 * If it got disabled meanwhile, bail out:
+			 */
+			if (atomic_read(&t->count))
+				goto out_disabled;
+			/*
+			 * If it got scheduled meanwhile, re-execute
+			 * the tasklet function:
+			 */
+			if (test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
+				goto again;
+			if (!--loops) {
+				printk("hm, tasklet state: %08lx\n", t->state);
+				WARN_ON(1);
+				tasklet_unlock(t);
+				break;
+			}
+		}
 	}
 }
 
-static void tasklet_hi_action(struct softirq_action *a)
+static void tasklet_action(struct softirq_action *a)
 {
 	struct tasklet_struct *list;
 
 	local_irq_disable();
-	list = __get_cpu_var(tasklet_hi_vec).head;
-	__get_cpu_var(tasklet_hi_vec).head = NULL;
-	__get_cpu_var(tasklet_hi_vec).tail = &__get_cpu_var(tasklet_hi_vec).head;
+	list = __get_cpu_var(tasklet_vec).head;
+	__get_cpu_var(tasklet_vec).head = NULL;
+	__get_cpu_var(tasklet_vec).tail = &__get_cpu_var(tasklet_vec).head;
 	local_irq_enable();
 
-	while (list) {
-		struct tasklet_struct *t = list;
+	__tasklet_action(a, list);
+}
 
-		list = list->next;
+static void tasklet_hi_action(struct softirq_action *a)
+{
+	struct tasklet_struct *list;
 
-		if (tasklet_trylock(t)) {
-			if (!atomic_read(&t->count)) {
-				if (!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
-					BUG();
-				t->func(t->data);
-				tasklet_unlock(t);
-				continue;
-			}
-			tasklet_unlock(t);
-		}
+	local_irq_disable();
+	list = __get_cpu_var(tasklet_hi_vec).head;
+	__get_cpu_var(tasklet_hi_vec).head = NULL;
+	__get_cpu_var(tasklet_hi_vec).tail = &__get_cpu_var(tasklet_vec).head;
+	local_irq_enable();
 
-		local_irq_disable();
-		t->next = NULL;
-		*__get_cpu_var(tasklet_hi_vec).tail = t;
-		__get_cpu_var(tasklet_hi_vec).tail = &(t->next);
-		__do_raise_softirq_irqoff(HI_SOFTIRQ);
-		local_irq_enable();
-	}
+	__tasklet_action(a, list);
 }
 
 
-- 
1.7.1.1