[PATCH] timer race fixes

From: Ingo Molnar <mingo@elte.hu>

It unifies the functionality of add_timer() and mod_timer(), and makes any
combination of the timer API calls completely SMP-safe.  del_timer() is still
not using the timer lock.

this patch fixes the only timer bug in 2.6 i'm aware of: the del_timer_sync()
+ add_timer() combination in kernel/itimer.c is buggy.  This was correct code
in 2.4, because there it was safe to do an add_timer() from the timer handler
itself, parallel to a del_timer_sync().

If we want to make this safe in 2.6 too (which i think we want to) then we
have to make add_timer() almost equivalent to mod_timer(), locking-wise.  And
once we are at this point i think it's much cleaner to actually make
add_timer() a variant of mod_timer().  (There's no locking cost for
add_timer(), only the cost of an extra branch.  And we've removed another
commonly used function from the icache.)

1 files changed, 21 insertions, 2 deletions

diff --git a/include/linux/timer.h b/include/linux/timer.h
index d98c9a948aa7..e8f79ea49c74 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -60,11 +60,30 @@ static inline int timer_pending(const struct timer_list * timer)
 	return timer->base != NULL;
 }
 
-extern void add_timer(struct timer_list * timer);
 extern void add_timer_on(struct timer_list *timer, int cpu);
 extern int del_timer(struct timer_list * timer);
+extern int __mod_timer(struct timer_list *timer, unsigned long expires);
 extern int mod_timer(struct timer_list *timer, unsigned long expires);
-  
+
+/***
+ * add_timer - start a timer
+ * @timer: the timer to be added
+ *
+ * The kernel will do a ->function(->data) callback from the
+ * timer interrupt at the ->expired point in the future. The
+ * current time is 'jiffies'.
+ *
+ * The timer's ->expired, ->function (and if the handler uses it, ->data)
+ * fields must be set prior calling this function.
+ *
+ * Timers with an ->expired field in the past will be executed in the next
+ * timer tick.
+ */
+static inline void add_timer(struct timer_list * timer)
+{
+	__mod_timer(timer, timer->expires);
+}
+
 #ifdef CONFIG_SMP
   extern int del_timer_sync(struct timer_list * timer);
 #else