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SubjectRe: too much timer simplification...
Andrew Morton wrote:
> David Mosberger <davidm@napali.hpl.hp.com> 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.
>
I think the answer here is to move the whole expired list to a local
header and to not look back. I will work this into a patch...

-g

> 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);
>
> _
>
>

--
George Anzinger george@mvista.com
High-res-timers: http://sourceforge.net/projects/high-res-timers/
Preemption patch: http://www.kernel.org/pub/linux/kernel/people/rml

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