[PATCH -tip] Simplify RCU CPU-hotplug notification

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

Simplify RCU's CPU-hotplug notifiers so that there is only one notifier.
This makes it trivial to provide the ordering guarantee that rcu_barrier()
depends on.  This change also allows rcu_barrier() to rely on locking
wholly contained in _cpu_down(), rather than that of its callers, making
the locking more straightforward.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---

 rcupdate.c   |    8 ++++++--
 rcupreempt.c |   22 +++++-----------------
 rcutree.c    |   18 +++++++++---------
 3 files changed, 20 insertions(+), 28 deletions(-)

diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index eae29c2..3fea910 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -217,9 +217,13 @@ static void rcu_migrate_callback(struct rcu_head *notused)
 		wake_up(&rcu_migrate_wq);
 }
 
+extern int rcu_cpu_notify(struct notifier_block *self,
+			  unsigned long action, void *hcpu);
+
 static int __cpuinit rcu_barrier_cpu_hotplug(struct notifier_block *self,
 		unsigned long action, void *hcpu)
 {
+	rcu_cpu_notify(self, action, hcpu);
 	if (action == CPU_DYING) {
 		/*
 		 * preempt_disable() in on_each_cpu() prevents stop_machine(),
@@ -234,7 +238,7 @@ static int __cpuinit rcu_barrier_cpu_hotplug(struct notifier_block *self,
 		call_rcu_bh(rcu_migrate_head, rcu_migrate_callback);
 		call_rcu_sched(rcu_migrate_head + 1, rcu_migrate_callback);
 		call_rcu(rcu_migrate_head + 2, rcu_migrate_callback);
-	} else if (action == CPU_POST_DEAD) {
+	} else if (action == CPU_DEAD) {
 		/* rcu_migrate_head is protected by cpu_add_remove_lock */
 		wait_migrated_callbacks();
 	}
@@ -244,8 +248,8 @@ static int __cpuinit rcu_barrier_cpu_hotplug(struct notifier_block *self,
 
 void __init rcu_init(void)
 {
-	__rcu_init();
 	hotcpu_notifier(rcu_barrier_cpu_hotplug, 0);
+	__rcu_init();
 }
 
 void rcu_scheduler_starting(void)
diff --git a/kernel/rcupreempt.c b/kernel/rcupreempt.c
index beb0e65..4300212 100644
--- a/kernel/rcupreempt.c
+++ b/kernel/rcupreempt.c
@@ -1417,7 +1417,7 @@ int rcu_pending(int cpu)
 	return 0;
 }
 
-static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
+int __cpuinit rcu_cpu_notify(struct notifier_block *self,
 				unsigned long action, void *hcpu)
 {
 	long cpu = (long)hcpu;
@@ -1439,10 +1439,6 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
 	return NOTIFY_OK;
 }
 
-static struct notifier_block __cpuinitdata rcu_nb = {
-	.notifier_call = rcu_cpu_notify,
-};
-
 void __init __rcu_init(void)
 {
 	int cpu;
@@ -1471,22 +1467,14 @@ void __init __rcu_init(void)
 		rdp->waitschedtail = &rdp->waitschedlist;
 		rdp->rcu_sched_sleeping = 0;
 	}
-	register_cpu_notifier(&rcu_nb);
 
 	/*
-	 * We don't need protection against CPU-Hotplug here
-	 * since
-	 * a) If a CPU comes online while we are iterating over the
-	 *    cpu_online_mask below, we would only end up making a
-	 *    duplicate call to rcu_online_cpu() which sets the corresponding
-	 *    CPU's mask in the rcu_cpu_online_map.
-	 *
-	 * b) A CPU cannot go offline at this point in time since the user
-	 *    does not have access to the sysfs interface, nor do we
-	 *    suspend the system.
+	 * We don't need protection against CPU-hotplug here because
+	 * this is called early in boot, before either interrupts
+	 * or the scheduler are operational.
 	 */
 	for_each_online_cpu(cpu)
-		rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE,	(void *)(long) cpu);
+		rcu_cpu_notify(NULL, CPU_UP_PREPARE, (void *)(long) cpu);
 
 	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
 }
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 7717b95..e8e9e93 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1407,13 +1407,12 @@ static void __cpuinit rcu_online_cpu(int cpu)
 {
 	rcu_init_percpu_data(cpu, &rcu_state);
 	rcu_init_percpu_data(cpu, &rcu_bh_state);
-	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
 }
 
 /*
  * Handle CPU online/offline notifcation events.
  */
-static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
+int __cpuinit rcu_cpu_notify(struct notifier_block *self,
 				unsigned long action, void *hcpu)
 {
 	long cpu = (long)hcpu;
@@ -1523,10 +1522,6 @@ do { \
 	} \
 } while (0)
 
-static struct notifier_block __cpuinitdata rcu_nb = {
-	.notifier_call	= rcu_cpu_notify,
-};
-
 void __init __rcu_init(void)
 {
 	int i;			/* All used by RCU_DATA_PTR_INIT(). */
@@ -1542,10 +1537,15 @@ void __init __rcu_init(void)
 	rcu_init_one(&rcu_bh_state);
 	RCU_DATA_PTR_INIT(&rcu_bh_state, rcu_bh_data);
 
+	/*
+	 * We don't need protection against CPU-hotplug here because
+	 * this is called early in boot, before either interrupts
+	 * or the scheduler are operational.
+	 */
 	for_each_online_cpu(i)
-		rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE, (void *)(long)i);
-	/* Register notifier for non-boot CPUs */
-	register_cpu_notifier(&rcu_nb);
+		rcu_cpu_notify(NULL, CPU_UP_PREPARE, (void *)(long)i);
+
+	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
 }
 
 module_param(blimit, int, 0);