On Tuesday 22 January 2008 02:22, Steven Rostedt wrote:
> From: john stultz <[EMAIL PROTECTED]>
> 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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