[PATCH rc4-mm2 1/2] timers: introduce try_to_del_timer_sync()

[Oleg Nesterov <oleg@tv-sign.ru>]

This patch splits del_timer_sync() into 2 functions. The new one,
try_to_del_timer_sync(), returns -1 when it hits executing timer.

It can be used in interrupt context, or when the caller hold locks
which can prevent completion of the timer's handler.

NOTE. Currently it can't be used in interrupt context in UP case,
because ->running_timer is used only with CONFIG_SMP.

Should the need arise, it is possible to kill #ifdef CONFIG_SMP in
set_running_timer(), it is cheap.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- 2.6.12-rc4-mm2/include/linux/timer.h~1_TRY	2005-05-22 15:41:51.000000000 +0400
+++ 2.6.12-rc4-mm2/include/linux/timer.h	2005-05-22 17:34:00.000000000 +0400
@@ -76,9 +76,11 @@ static inline void add_timer(struct time
 }
 
 #ifdef CONFIG_SMP
+  extern int try_to_del_timer_sync(struct timer_list *timer);
   extern int del_timer_sync(struct timer_list *timer);
 #else
-# define del_timer_sync(t) del_timer(t)
+# define try_to_del_timer_sync(t)	del_timer(t)
+# define del_timer_sync(t)		del_timer(t)
 #endif
 
 #define del_singleshot_timer_sync(t) del_timer_sync(t)
--- 2.6.12-rc4-mm2/kernel/timer.c~1_TRY	2005-05-22 15:41:51.000000000 +0400
+++ 2.6.12-rc4-mm2/kernel/timer.c	2005-05-22 18:08:15.000000000 +0400
@@ -366,6 +366,34 @@ int del_timer(struct timer_list *timer)
 EXPORT_SYMBOL(del_timer);
 
 #ifdef CONFIG_SMP
+/*
+ * This function tries to deactivate a timer. Upon successful (ret >= 0)
+ * exit the timer is not queued and the handler is not running on any CPU.
+ *
+ * It must not be called from interrupt contexts.
+ */
+int try_to_del_timer_sync(struct timer_list *timer)
+{
+	timer_base_t *base;
+	unsigned long flags;
+	int ret = -1;
+
+	base = lock_timer_base(timer, &flags);
+
+	if (base->running_timer == timer)
+		goto out;
+
+	ret = 0;
+	if (timer_pending(timer)) {
+		detach_timer(timer, 1);
+		ret = 1;
+	}
+out:
+	spin_unlock_irqrestore(&base->lock, flags);
+
+	return ret;
+}
+
 /***
  * del_timer_sync - deactivate a timer and wait for the handler to finish.
  * @timer: the timer to be deactivated
@@ -385,28 +413,13 @@ EXPORT_SYMBOL(del_timer);
  */
 int del_timer_sync(struct timer_list *timer)
 {
-	timer_base_t *base;
-	unsigned long flags;
-	int ret = -1;
-
 	check_timer(timer);
 
-	do {
-		base = lock_timer_base(timer, &flags);
-
-		if (base->running_timer == timer)
-			goto unlock;
-
-		ret = 0;
-		if (timer_pending(timer)) {
-			detach_timer(timer, 1);
-			ret = 1;
-		}
-unlock:
-		spin_unlock_irqrestore(&base->lock, flags);
-	} while (ret < 0);
-
-	return ret;
+	for (;;) {
+		int ret = try_to_del_timer_sync(timer);
+		if (ret >= 0)
+			return ret;
+	}
 }
 
 EXPORT_SYMBOL(del_timer_sync);