Re: [PATCH RFC nohz_full 6/7] nohz_full: Add full-system-idle state machine

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Jul 08, 2013 at 06:30:05PM -0700, Paul E. McKenney wrote:
>  }
>  
>  /*
> + * Unconditionally force exit from full system-idle state.  This is
> + * invoked when a normal CPU exits idle, but must be called separately
> + * for the timekeeping CPU (tick_do_timer_cpu).  The reason for this
> + * is that the timekeeping CPU is permitted to take scheduling-clock
> + * interrupts while the system is in system-idle state, and of course
> + * rcu_sysidle_exit() has no way of distinguishing a scheduling-clock
> + * interrupt from any other type of interrupt.
> + */
> +void rcu_sysidle_force_exit(void)
> +{
> +	int oldstate = ACCESS_ONCE(full_sysidle_state);
> +	int newoldstate;
> +
> +	/*
> +	 * Each pass through the following loop attempts to exit full
> +	 * system-idle state.  If contention proves to be a problem,
> +	 * a trylock-based contention tree could be used here.
> +	 */
> +	while (oldstate > RCU_SYSIDLE_SHORT) {

I'm missing a key here.

Let's imagine that the timekeeper has finally set full_sysidle_state = RCU_SYSIDLE_FULL_NOTED
with cmpxchg, what guarantees that this CPU is not seeing a stale RCU_SYSIDLE_SHORT value
for example?