simplify cpu_hotplug_begin()/put_online_cpus()

cpu_hotplug_begin() must be always called under cpu_add_remove_lock, this
means that only one process can be cpu_hotplug.active_writer.  So we don't
need the cpu_hotplug.writer_queue, we can wake up the ->active_writer
directly.

Also, fix the comment.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Dipankar Sarma <dipankar@in.ibm.com>
Acked-by: Gautham R Shenoy <ego@in.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/cpu.c

@@ -33,17 +33,13 @@ static struct {
 	 * an ongoing cpu hotplug operation.
 	 */
 	int refcount;
-	wait_queue_head_t writer_queue;
 } cpu_hotplug;
 
-#define writer_exists() (cpu_hotplug.active_writer != NULL)
-
 void __init cpu_hotplug_init(void)
 {
 	cpu_hotplug.active_writer = NULL;
 	mutex_init(&cpu_hotplug.lock);
 	cpu_hotplug.refcount = 0;
-	init_waitqueue_head(&cpu_hotplug.writer_queue);
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
@@ -65,11 +61,8 @@ void put_online_cpus(void)
 	if (cpu_hotplug.active_writer == current)
 		return;
 	mutex_lock(&cpu_hotplug.lock);
-	cpu_hotplug.refcount--;
-
-	if (unlikely(writer_exists()) && !cpu_hotplug.refcount)
-		wake_up(&cpu_hotplug.writer_queue);
-
+	if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
+		wake_up_process(cpu_hotplug.active_writer);
 	mutex_unlock(&cpu_hotplug.lock);
 
 }
@@ -98,8 +91,8 @@ void cpu_maps_update_done(void)
  * Note that during a cpu-hotplug operation, the new readers, if any,
  * will be blocked by the cpu_hotplug.lock
  *
- * Since cpu_maps_update_begin is always called after invoking
- * cpu_maps_update_begin, we can be sure that only one writer is active.
+ * 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
@@ -115,19 +108,16 @@ void cpu_maps_update_done(void)
  */
 static void cpu_hotplug_begin(void)
 {
-	DECLARE_WAITQUEUE(wait, current);
+	cpu_hotplug.active_writer = current;
 
+	for (;;) {
 		mutex_lock(&cpu_hotplug.lock);
-
-	cpu_hotplug.active_writer = current;
-	add_wait_queue_exclusive(&cpu_hotplug.writer_queue, &wait);
-	while (cpu_hotplug.refcount) {
-		set_current_state(TASK_UNINTERRUPTIBLE);
+		if (likely(!cpu_hotplug.refcount))
+			break;
+		__set_current_state(TASK_UNINTERRUPTIBLE);
 		mutex_unlock(&cpu_hotplug.lock);
 		schedule();
-		mutex_lock(&cpu_hotplug.lock);
 	}
-	remove_wait_queue_locked(&cpu_hotplug.writer_queue, &wait);
 }
 
 static void cpu_hotplug_done(void)