On Tue, 22 Jan 2008, Nick Piggin wrote:
> 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.
Yep, thanks for the reminder. This was a quick patch from John. I'll add
the necessary comment for the next release. Hmm, I'm surprised that
checkpatch didn't complain.
>
>
> > + 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().
Yep, this too. We suggested earlier to John to switch this to a
smp_wmb, but I took his patch before the update. This will be fixed in the
next round too.
Thanks,
-- Steve
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