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 <paul...@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
>
