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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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