Subject: Re: [patch] timers: add mod_timer_pending()

From: Oleg Nesterov <[email-masked]>
Date: Wed, 18 Feb 2009 18:00:57 +0100

On 02/18, Ingo Molnar wrote:
>
> Based on an idea from Stephen Hemminger: introduce
>  mod_timer_pending() which is a mod_timer() offspring
> that is an invariant on already removed timers.

This also can be used by workqueues, see

	http://marc.info/?l=linux-kernel&m=122209752020413

but can't we add another helper? Because,

> +static inline int
> +__mod_timer(struct timer_list *timer, unsigned long expires, bool pending_only)
>  {
>  	struct tvec_base *base, *new_base;
>  	unsigned long flags;
> -	int ret = 0;
> +	int ret;
> +
> +	ret = 0;
>
>  	timer_stats_timer_set_start_info(timer);
>  	BUG_ON(!timer->function);
> @@ -614,6 +617,9 @@ int __mod_timer(struct timer_list *timer
>  	if (timer_pending(timer)) {
>  		detach_timer(timer, 0);
>  		ret = 1;
> +	} else {
> +		if (pending_only)
> +			goto out_unlock;

This can change the base (CPU) of the pending timer.

How about

	int __update_timer(struct timer_list *timer, unsigned long expires)
	{
		struct tvec_base *base;
		unsigned long flags;
		int ret = 0;

		base = lock_timer_base(timer, &flags);
		if (timer_pending(timer)) {
			detach_timer(timer, 0);
			timer->expires = expires;
			internal_add_timer(base, timer);
			ret = 1;
		}
		spin_unlock_irqrestore(&base->lock, flags);

		return ret;
	}

?

Unlike __mod_timer(..., bool pending_only), it preserves the CPU on
which the timer is pending.

Or, perhaps, we can modify __mod_timer() further,

	static inline int
	__mod_timer(struct timer_list *timer, unsigned long expires, bool pending_only)
	{
		struct tvec_base *base;
		unsigned long flags;
		int ret;

		ret = 0;

		timer_stats_timer_set_start_info(timer);
		BUG_ON(!timer->function);

		base = lock_timer_base(timer, &flags);

		if (timer_pending(timer)) {
			detach_timer(timer, 0);
			ret = 1;
		} else if (pending_only)
			goto out_unlock;
		}

		debug_timer_activate(timer);

		if (!pending_only) {
			struct tvec_base *new_base = __get_cpu_var(tvec_bases);

			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);

		return ret;
	}

What do you all think?

Oleg.