Re: too much timer simplification...

[Andrew Morton]

David Mosberger <[e-mail]> wrote:
>
> It appears to me that this changeset:
> 
>   http://linux.bkbits.net:8080/linux-2.5/diffs/kernel/timer.c@1.48
> 
> may have gone a little too far.
> 
> What I'm seeing is that if someone happens to arm a periodic timer at
> exactly 256 jiffies (as ohci happens to do on platforms with HZ=1024),
> then you end up getting an endless loop of timer activations, causing
> a machine hang.
> 
> The problem is that __run_timers updates base->timer_jiffies _before_
> running the callback routines.  If a callback re-arms the timer at
> exactly 256 jiffies, add_timers() will reinsert the timer into the
> list that we're currently processing, which of course will cause the
> timer to expire immediately again, etc., etc., ad naseum...
> 

OK, well unless George can pull a rabbit out of the hat it may
be best to just revert it.

This gives us the same algorithm as 2.4, which I think is good.

--- 25/kernel/timer.c~timer-simplification-revert	2003-04-11 00:19:48.000000000 -0700
+++ 25-akpm/kernel/timer.c	2003-04-11 00:19:48.000000000 -0700
@@ -56,6 +56,7 @@ struct tvec_t_base_s {
 	spinlock_t lock;
 	unsigned long timer_jiffies;
 	struct timer_list *running_timer;
+	struct list_head *run_timer_list_running;
 	tvec_root_t tv1;
 	tvec_t tv2;
 	tvec_t tv3;
@@ -100,6 +101,12 @@ static inline void check_timer(struct ti
 		check_timer_failed(timer);
 }
 
+/*
+ * If a timer handler re-adds the timer with expires == jiffies, the timer
+ * running code can lock up.  So here we detect that situation and park the
+ * timer onto base->run_timer_list_running.  It will be added to the main timer
+ * structures later, by __run_timers().
+ */
 
 static void internal_add_timer(tvec_base_t *base, struct timer_list *timer)
 {
@@ -107,7 +114,9 @@ static void internal_add_timer(tvec_base
 	unsigned long idx = expires - base->timer_jiffies;
 	struct list_head *vec;
 
-	if (idx < TVR_SIZE) {
+	if (base->run_timer_list_running) {
+		vec = base->run_timer_list_running;
+	} else if (idx < TVR_SIZE) {
 		int i = expires & TVR_MASK;
 		vec = base->tv1.vec + i;
 	} else if (idx < 1 << (TVR_BITS + TVN_BITS)) {
@@ -397,6 +406,7 @@ static inline void __run_timers(tvec_bas
 
 	spin_lock_irq(&base->lock);
 	while (time_after_eq(jiffies, base->timer_jiffies)) {
+		LIST_HEAD(deferred_timers);
 		struct list_head *head;
  		int index = base->timer_jiffies & TVR_MASK;
  
@@ -408,7 +418,7 @@ static inline void __run_timers(tvec_bas
 				(!cascade(base, &base->tv3, INDEX(1))) &&
 					!cascade(base, &base->tv4, INDEX(2)))
 			cascade(base, &base->tv5, INDEX(3));
-		++base->timer_jiffies; 
+		base->run_timer_list_running = &deferred_timers;
 repeat:
 		head = base->tv1.vec + index;
 		if (!list_empty(head)) {
@@ -427,6 +437,14 @@ repeat:
 			spin_lock_irq(&base->lock);
 			goto repeat;
 		}
+		base->run_timer_list_running = NULL;
+		++base->timer_jiffies; 
+		while (!list_empty(&deferred_timers)) {
+			timer = list_entry(deferred_timers.prev,
+						struct timer_list, entry);
+			list_del(&timer->entry);
+			internal_add_timer(base, timer);
+		}
 	}
 	set_running_timer(base, NULL);
 	spin_unlock_irq(&base->lock);

_