On Mon, Jul 01, 2013 at 06:53:57PM +0200, Frederic Weisbecker wrote:
> On Fri, Jun 28, 2013 at 01:10:21PM -0700, Paul E. McKenney wrote:
> > +
> > +/*
> > + * Check to see if the system is fully idle, other than the timekeeping
> > CPU.
> > + * The caller must have disabled interrupts.
> > + */
> > +bool rcu_sys_is_idle(void)
> > +{
> > + static struct rcu_sysidle_head rsh;
> > + int rss = ACCESS_ONCE(full_sysidle_state);
> > +
> > + WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu);
> > +
> > + /* Handle small-system case by doing a full scan of CPUs. */
> > + if (nr_cpu_ids <= RCU_SYSIDLE_SMALL && rss < RCU_SYSIDLE_FULL) {
> > + int cpu;
> > + bool isidle = true;
> > + unsigned long maxj = jiffies - ULONG_MAX / 4;
> > + struct rcu_data *rdp;
> > +
> > + /* Scan all the CPUs looking for nonidle CPUs. */
> > + for_each_possible_cpu(cpu) {
> > + rdp = per_cpu_ptr(rcu_sysidle_state->rda, cpu);
> > + rcu_sysidle_check_cpu(rdp, &isidle, &maxj);
> > + if (!isidle)
> > + break;
> > + }
> > + rcu_sysidle_report(rcu_sysidle_state, isidle, maxj);
>
> To clarify my worries, I would expect the following ordering:
>
> CPU 0 CPU 1
>
> cmpxchg(global_state, RCU_SYSIDLE_..., ...) write
> rdtp(1)->idle_dynticks
> smp_mb() // implied by cmpxchg smp_mb()
> read rdtp(1)-idle_dynticks cmpxchg(global_state,
> RCU_SYSIDLE_NONE)
>
> This example doesn't really make sense because CPU 0 only wants to change
> global state after
> checking rdtp(1)->idle_dynticks. So we obviously want the other way around as
> you did. But then
> I can't find an ordering that makes sure that only either happens:
>
> * CPU 0 sees CPU 1 update when it wakes up and so we reset to
> RCU_SYSIDLE_NONE from CPU 0
> * CPU 1 wakes up and reset to RCU_SYSIDLE_NONE and sends an IPI to CPU 0
>
> I mean that's what the code does, but I don't get the ordering that
> guarantees we can't
> fall into some intermediate state when CPU 0 sees CPU 1 idle whereas it
> already woke up without
> sending the IPI.
>
> I'm sure you'll point me to my mistaken review :)
Not at all -- as noted in previous email, you have helped me understand
why my original design included an RCU_SYSIDLE_LONG state.
Thanx, Paul
> > + rss = ACCESS_ONCE(full_sysidle_state);
> > + }
> > +
> > + /* If this is the first observation of an idle period, record it. */
> > + if (rss == RCU_SYSIDLE_FULL) {
> > + rss = cmpxchg(&full_sysidle_state,
> > + RCU_SYSIDLE_FULL, RCU_SYSIDLE_FULL_NOTED);
> > + return rss == RCU_SYSIDLE_FULL;
> > + }
> > +
> > + smp_mb(); /* ensure rss load happens before later caller actions. */
> > +
> > + /* If already fully idle, tell the caller (in case of races). */
> > + if (rss == RCU_SYSIDLE_FULL_NOTED)
> > + return true;
> > +
> > + /*
> > + * If we aren't there yet, and a grace period is not in flight,
> > + * initiate a grace period. Either way, tell the caller that
> > + * we are not there yet.
> > + */
> > + if (nr_cpu_ids > RCU_SYSIDLE_SMALL &&
> > + !rcu_gp_in_progress(rcu_sysidle_state) &&
> > + !rsh.inuse && xchg(&rsh.inuse, 1) == 0)
> > + call_rcu(&rsh.rh, rcu_sysidle_cb);
> > + return false;
> > }
> >
> > /*
> > @@ -2494,6 +2734,21 @@ static void rcu_sysidle_exit(struct rcu_dynticks
> > *rdtp, int irq)
> > {
> > }
> >
> > +static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
> > + unsigned long *maxj)
> > +{
> > +}
> > +
> > +static bool is_sysidle_rcu_state(struct rcu_state *rsp)
> > +{
> > + return false;
> > +}
> > +
> > +static void rcu_sysidle_report(struct rcu_state *rsp, int isidle,
> > + unsigned long maxj)
> > +{
> > +}
> > +
> > static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
> > {
> > }
> > --
> > 1.8.1.5
> >
>
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