[PATCH 1/2] cpuhotplug: use rw_semaphore for cpu_hotplug

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Current get_online_cpus()/put_online_cpus() re-implement
a rw_semaphore, so it is converted to a real rw_semaphore in this fix.
It simplifies codes, and is good for read.

And misc fix:
1) Add comments for cpu_hotplug.active_writer.
2) The theoretical disadvantage described in cpu_hotplug_begin()'s
   comments is no longer existed when we use rw_semaphore,
   so this part of comments was removed.

[Impact: improve get_online_cpus()/put_online_cpus() ]

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 395b697..62198ec 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -14,6 +14,7 @@
 #include <linux/kthread.h>
 #include <linux/stop_machine.h>
 #include <linux/mutex.h>
+#include <linux/rwsem.h>
 
 #ifdef CONFIG_SMP
 /* Serializes the updates to cpu_online_mask, cpu_present_mask */
@@ -27,20 +28,21 @@ static __cpuinitdata RAW_NOTIFIER_HEAD(cpu_chain);
 static int cpu_hotplug_disabled;
 
 static struct {
-	struct task_struct *active_writer;
-	struct mutex lock; /* Synchronizes accesses to refcount, */
 	/*
-	 * Also blocks the new readers during
-	 * an ongoing cpu hotplug operation.
+	 * active_writer makes get_online_cpus()/put_online_cpus() are allowd
+	 * to be nested in cpu_hotplug_begin()/cpu_hotplug_done().
+	 *
+	 * Thus, get_online_cpus()/put_online_cpus() can be called in
+	 * CPU notifiers.
 	 */
-	int refcount;
+	struct task_struct *active_writer;
+	struct rw_semaphore rwlock;
 } cpu_hotplug;
 
 void __init cpu_hotplug_init(void)
 {
 	cpu_hotplug.active_writer = NULL;
-	mutex_init(&cpu_hotplug.lock);
-	cpu_hotplug.refcount = 0;
+	init_rwsem(&cpu_hotplug.rwlock);
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
@@ -50,9 +52,7 @@ void get_online_cpus(void)
 	might_sleep();
 	if (cpu_hotplug.active_writer == current)
 		return;
-	mutex_lock(&cpu_hotplug.lock);
-	cpu_hotplug.refcount++;
-	mutex_unlock(&cpu_hotplug.lock);
+	down_read(&cpu_hotplug.rwlock);
 
 }
 EXPORT_SYMBOL_GPL(get_online_cpus);
@@ -61,10 +61,7 @@ void put_online_cpus(void)
 {
 	if (cpu_hotplug.active_writer == current)
 		return;
-	mutex_lock(&cpu_hotplug.lock);
-	if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
-		wake_up_process(cpu_hotplug.active_writer);
-	mutex_unlock(&cpu_hotplug.lock);
+	up_read(&cpu_hotplug.rwlock);
 
 }
 EXPORT_SYMBOL_GPL(put_online_cpus);
@@ -86,45 +83,25 @@ void cpu_maps_update_done(void)
 }
 
 /*
- * This ensures that the hotplug operation can begin only when the
- * refcount goes to zero.
+ * This ensures that the hotplug operation can begin only when
+ * there is no reader.
  *
  * Note that during a cpu-hotplug operation, the new readers, if any,
- * will be blocked by the cpu_hotplug.lock
+ * will be blocked by the cpu_hotplug.rwlock
  *
  * Since cpu_hotplug_begin() is always called after invoking
  * cpu_maps_update_begin(), we can be sure that only one writer is active.
- *
- * Note that theoretically, there is a possibility of a livelock:
- * - Refcount goes to zero, last reader wakes up the sleeping
- *   writer.
- * - Last reader unlocks the cpu_hotplug.lock.
- * - A new reader arrives at this moment, bumps up the refcount.
- * - The writer acquires the cpu_hotplug.lock finds the refcount
- *   non zero and goes to sleep again.
- *
- * However, this is very difficult to achieve in practice since
- * get_online_cpus() not an api which is called all that often.
- *
  */
 static void cpu_hotplug_begin(void)
 {
+	down_write(&cpu_hotplug.rwlock);
 	cpu_hotplug.active_writer = current;
-
-	for (;;) {
-		mutex_lock(&cpu_hotplug.lock);
-		if (likely(!cpu_hotplug.refcount))
-			break;
-		__set_current_state(TASK_UNINTERRUPTIBLE);
-		mutex_unlock(&cpu_hotplug.lock);
-		schedule();
-	}
 }
 
 static void cpu_hotplug_done(void)
 {
 	cpu_hotplug.active_writer = NULL;
-	mutex_unlock(&cpu_hotplug.lock);
+	up_write(&cpu_hotplug.rwlock);
 }
 /* Need to know about CPUs going up/down? */
 int __ref register_cpu_notifier(struct notifier_block *nb)