[PATCH v2] timers: fix synchronization rules in comments of del_timer_sync

[Duoming Zhou]

The del_timer_sync() could stop the periodic timer. The root cause
is shown below which is a part of code in del_timer_sync:

	do {
		ret = try_to_del_timer_sync(timer);

		if (unlikely(ret < 0)) {
			del_timer_wait_running(timer);
			cpu_relax();
		}
	} while (ret < 0);

If the timer's handler is running, the try_to_del_timer_sync will
return -1. Then, it will loop until the timer is not queued and
the timer's handler is not running on any CPU.

Although the periodic timer may restart itself in timer's handler,
the del_timer_sync could also stop it.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Duoming Zhou <duoming@xxxxxxxxxx>
---
Changes in v2:
  - Make commit messages clearer.
  - Change the annotations about del_timer_sync.

 kernel/time/timer.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 717fcb9fb14..dd623018dbc 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1375,11 +1375,11 @@ static inline void del_timer_wait_running(struct timer_list *timer) { }
  * CPUs.
  *
  * Synchronization rules: Callers must prevent restarting of the timer,
- * otherwise this function is meaningless. It must not be called from
- * interrupt contexts unless the timer is an irqsafe one. The caller must
- * not hold locks which would prevent completion of the timer's
- * handler. The timer's handler must not call add_timer_on(). Upon exit the
- * timer is not queued and the handler is not running on any CPU.
+ * otherwise this function is meaningless. It could also stop periodic timer.
+ * It must not be called from interrupt contexts unless the timer is an irqsafe
+ * one. The caller must not hold locks which would prevent completion of the
+ * timer's handler. Upon exit the timer is not queued and the handler is not
+ * running on any CPU.
  *
  * Note: For !irqsafe timers, you must not hold locks that are held in
  *   interrupt context while calling this function. Even if the lock has
-- 
2.17.1