Re: [RFC PATCH 12/23 -v4] Use RCU algorithm for monotonic cycles.

[Nick Piggin]

On Tuesday 22 January 2008 02:22, Steven Rostedt wrote:
> From: john stultz <johnstul@xxxxxxxxxx>

>  static inline cycle_t
> -clocksource_get_cycles(struct clocksource *cs, cycle_t now)
> +clocksource_get_basecycles(struct clocksource *cs)
>  {
> -	cycle_t offset = (now - cs->cycle_last) & cs->mask;
> -	offset += cs->cycle_accumulated;
> +	int num;
> +	cycle_t now, offset;
> +
> +	preempt_disable();
> +	num = cs->base_num;
> +	smp_read_barrier_depends();

All barriers need comments in the code. eg. with read barriers, the
comment should contain a list of the loads being ordered, and a
reference to the places where stores come from.

I know it isn't too hard to follow _now_, but it makes the code more
maintainable.


> +	now = clocksource_read(cs);
> +	offset = (now - cs->base[num].cycle_base_last);
> +	offset &= cs->mask;
> +	offset += cs->base[num].cycle_base;
> +	preempt_enable();
> +
>  	return offset;
>  }
>
> @@ -197,14 +215,26 @@ clocksource_get_cycles(struct clocksourc
>   * @now:	current cycle value
>   *
>   * Used to avoids clocksource hardware overflow by periodically
> - * accumulating the current cycle delta. Must hold xtime write lock!
> + * accumulating the current cycle delta. Uses RCU-like update, but
> + * ***still requires the xtime_lock is held for writing!***
>   */
>  static inline void clocksource_accumulate(struct clocksource *cs, cycle_t
> now) {
> -	cycle_t offset = (now - cs->cycle_last) & cs->mask;
> +	/* First update the monotonic base portion.
> +	 * The dual array update method allows for lock-free reading.
> +	 */
> +	int num = 1 - cs->base_num;
> +	cycle_t offset = (now - cs->base[1-num].cycle_base_last);
> +	offset &= cs->mask;
> +	cs->base[num].cycle_base = cs->base[1-num].cycle_base + offset;
> +	cs->base[num].cycle_base_last = now;
> +	wmb();
> +	cs->base_num = num;

Ditto for the wmb. Also, I think the wmb() can probably just be
smp_wmb().
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