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

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)



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