Re: [PATCH tip/core/rcu 1/6] rcu: Abstract the dynticks momentary-idle operation

[Josh Triplett <josh@joshtriplett.org>]

On Sat, Jan 14, 2017 at 12:54:40AM -0800, Paul E. McKenney wrote:
> This commit is the first step towards full abstraction of all accesses to
> the ->dynticks counter, implementing the previously open-coded atomic add
> of two in a new rcu_dynticks_momentary_idle() function.  This abstraction
> will ease changes to the ->dynticks counter operation.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

This change has an additional effect not documented in the commit
message: it eliminates the smp_mb__before_atomic and
smp_mb__after_atomic calls.  Can you please document that in the commit
message, and explain why that doesn't cause a problem?

> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -281,6 +281,19 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
>  #endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
>  };
>  
> +/*
> + * Do a double-increment of the ->dynticks counter to emulate a
> + * momentary idle-CPU quiescent state.
> + */
> +static void rcu_dynticks_momentary_idle(void)
> +{
> +	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> +	int special = atomic_add_return(2, &rdtp->dynticks);
> +
> +	/* It is illegal to call this from idle state. */
> +	WARN_ON_ONCE(!(special & 0x1));
> +}
> +
>  DEFINE_PER_CPU_SHARED_ALIGNED(unsigned long, rcu_qs_ctr);
>  EXPORT_PER_CPU_SYMBOL_GPL(rcu_qs_ctr);
>  
> @@ -300,7 +313,6 @@ EXPORT_PER_CPU_SYMBOL_GPL(rcu_qs_ctr);
>  static void rcu_momentary_dyntick_idle(void)
>  {
>  	struct rcu_data *rdp;
> -	struct rcu_dynticks *rdtp;
>  	int resched_mask;
>  	struct rcu_state *rsp;
>  
> @@ -327,10 +339,7 @@ static void rcu_momentary_dyntick_idle(void)
>  		 * quiescent state, with no need for this CPU to do anything
>  		 * further.
>  		 */
> -		rdtp = this_cpu_ptr(&rcu_dynticks);
> -		smp_mb__before_atomic(); /* Earlier stuff before QS. */
> -		atomic_add(2, &rdtp->dynticks);  /* QS. */
> -		smp_mb__after_atomic(); /* Later stuff after QS. */
> +		rcu_dynticks_momentary_idle();
>  		break;
>  	}
>  }
> -- 
> 2.5.2
>