spin_lock---------------------spin_unlock------------------最轻

spin_lock_bh----------------spin_unlock_bh-----------------较轻

spin_lock_irq----------------spin_unlock_irq---------------较重

spin_lock_irqsave---------spin_lock_irqrestore--------------最重

static inline int

__mod_timer(struct timer_list *timer, unsigned long expires,

                        bool pending_only, int pinned)

{

    struct tvec_base *base, *new_base;

    unsigned long flags;

    int ret =  , cpu;

    timer_stats_timer_set_start_info(timer);

    BUG_ON(!timer->function);

    base = lock_timer_base(timer, &flags);

    ret = detach_if_pending(timer, base, false);

    if (!ret && pending_only)

        goto out_unlock;

    debug_activate(timer, expires);

    cpu = smp_processor_id();

#if defined(CONFIG_NO_HZ_COMMON) && defined(CONFIG_SMP)

    if (!pinned && get_sysctl_timer_migration())

        cpu = get_nohz_timer_target();

#endif

    new_base = per_cpu(tvec_bases, cpu);

    if (base != new_base) {

        /*

         * We are trying to schedule the timer on the local CPU.

         * However we can't change timer's base while it is running,

         * otherwise del_timer_sync() can't detect that the timer's

         * handler yet has not finished. This also guarantees that

         * the timer is serialized wrt itself.

         */

        if (likely(base->running_timer != timer)) {

            /* See the comment in lock_timer_base() */

            timer_set_base(timer, NULL);

            spin_unlock(&base->lock);

            base = new_base;

            spin_lock(&base->lock);

            timer_set_base(timer, base);

        }

    }

    timer->expires = expires;

    internal_add_timer(base, timer);

out_unlock:

    spin_unlock_irqrestore(&base->lock, flags);-----------收尾很重要

    return ret;

}