Re: task isolation discussion at Linux Plumbers

[Will Deacon]

Hi Paul,

Just a couple of comments, but they be more suited to Andy.

On Wed, Nov 09, 2016 at 09:38:08AM -0800, Paul E. McKenney wrote:
> @@ -355,10 +373,33 @@ static bool rcu_dynticks_in_eqs_since(struct rcu_dynticks *rdtp, int snap)
>  static void rcu_dynticks_momentary_idle(void)
>  {
>  	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> -	int special = atomic_add_return(2, &rdtp->dynticks);
> +	int special = atomic_add_return(2 * RCU_DYNTICK_CTRL_CTR,
> +					&rdtp->dynticks);
>  
>  	/* It is illegal to call this from idle state. */
> -	WARN_ON_ONCE(!(special & 0x1));
> +	WARN_ON_ONCE(!(special & RCU_DYNTICK_CTRL_CTR));
> +}
> +
> +/*
> + * Set the special (bottom) bit of the specified CPU so that it
> + * will take special action (such as flushing its TLB) on the
> + * next exit from an extended quiescent state.  Returns true if
> + * the bit was successfully set, or false if the CPU was not in
> + * an extended quiescent state.
> + */

Given that TLB maintenance on arm is handled in hardware (no need for IPI),
I'd like to avoid this work if at all possible. However, without seeing the
call site I can't tell if it's optional.

> +bool rcu_eqs_special_set(int cpu)
> +{
> +	int old;
> +	int new;
> +	struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
> +
> +	do {
> +		old = atomic_read(&rdtp->dynticks);
> +		if (old & RCU_DYNTICK_CTRL_CTR)
> +			return false;
> +		new = old | RCU_DYNTICK_CTRL_MASK;
> +	} while (atomic_cmpxchg(&rdtp->dynticks, old, new) != old);
> +	return true;
>  }

Can this be a cmpxchg_relaxed? What is it attempting to order?

Will