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    SubjectRe: [PATCH 1/9] timer locking optimization
    Hi,

    On Tue, 29 Nov 2005, Oleg Nesterov wrote:

    > Also, you have wrong value of 'base' after 'goto restart'.

    Here is the updated patch, which fixes this.

    bye, Roman

    Index: linux-2.6/kernel/timer.c
    ===================================================================
    --- linux-2.6.orig/kernel/timer.c 2005-11-29 13:29:35.000000000 +0100
    +++ linux-2.6/kernel/timer.c 2005-11-29 14:01:42.000000000 +0100
    @@ -178,27 +178,20 @@ static inline void detach_timer(struct t
    *
    * So __run_timers/migrate_timers can safely modify all timers which could
    * be found on ->tvX lists.
    - *
    - * When the timer's base is locked, and the timer removed from list, it is
    - * possible to set timer->base = NULL and drop the lock: the timer remains
    - * locked.
    */
    static timer_base_t *lock_timer_base(struct timer_list *timer,
    unsigned long *flags)
    {
    timer_base_t *base;

    - for (;;) {
    - base = timer->base;
    - if (likely(base != NULL)) {
    - spin_lock_irqsave(&base->lock, *flags);
    - if (likely(base == timer->base))
    - return base;
    - /* The timer has migrated to another CPU */
    - spin_unlock_irqrestore(&base->lock, *flags);
    - }
    - cpu_relax();
    - }
    +again:
    + base = timer->base;
    + spin_lock_irqsave(&base->lock, *flags);
    + if (likely(base == timer->base))
    + return base;
    + /* The timer has migrated to another CPU */
    + spin_unlock_irqrestore(&base->lock, *flags);
    + goto again;
    }

    int __mod_timer(struct timer_list *timer, unsigned long expires)
    @@ -210,6 +203,7 @@ int __mod_timer(struct timer_list *timer

    BUG_ON(!timer->function);

    +restart:
    base = lock_timer_base(timer, &flags);

    if (timer_pending(timer)) {
    @@ -231,11 +225,18 @@ int __mod_timer(struct timer_list *timer
    /* The timer remains on a former base */
    new_base = container_of(base, tvec_base_t, t_base);
    } else {
    - /* See the comment in lock_timer_base() */
    - timer->base = NULL;
    + /*
    + * We shortly release the timer and the timer can
    + * migrate to another cpu, so recheck the base after
    + * getting the lock.
    + */
    + timer->base = &new_base->t_base;
    spin_unlock(&base->lock);
    spin_lock(&new_base->t_base.lock);
    - timer->base = &new_base->t_base;
    + if (unlikely(timer->base != &new_base->t_base)) {
    + spin_unlock_irqrestore(&new_base->t_base.lock, flags);
    + goto restart;
    + }
    }
    }

    -
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      Last update: 2005-11-30 03:35    [W:0.028 / U:389.828 seconds]
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