linux-kernel - [PATCH] timers: fix synchronization rules in comments of del_timer

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Date:   Fri,  1 Jul 2022 16:55:35 +0800
From:   Duoming Zhou <duoming@....edu.cn>
To:     linux-kernel@...r.kernel.org
Cc:     jstultz@...gle.com, tglx@...utronix.de, sboyd@...nel.org,
        edumazet@...gle.com, Duoming Zhou <duoming@....edu.cn>
Subject: [PATCH] timers: fix synchronization rules in comments of del_timer_sync

The del_timer_sync() could stop the timer that restart itself
in the timer's handler. So the synchronization rules should be
changed to "Callers must prevent restarting of the timer in
other places except for the timer's handler".

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 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@....edu.cn>
---
 kernel/time/timer.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 717fcb9fb14..823e45c1235 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1374,12 +1374,13 @@ static inline void del_timer_wait_running(struct timer_list *timer) { }
  * the timer it also makes sure the handler has finished executing on other
  * 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.
+ * Synchronization rules: Callers must prevent restarting of the timer in
+ * other places except for the timer's handler, 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.
  *
  * 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