Re: [PATCH RFC v2 rcu 1/8] srcu: Convert ->srcu_lock_count and ->srcu_unlock_count to atomic

John Ogness
2022-09-30T15:03:26Z

Hi Paul,

On 2022-09-29, "Paul E. McKenney" <paulmck@kernel.org> wrote:
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 1c304fec89c0..6fd0665f4d1f 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -636,7 +636,7 @@ int __srcu_read_lock(struct srcu_struct *ssp)
> int idx;
>
> idx = READ_ONCE(ssp->srcu_idx) & 0x1;
> - this_cpu_inc(ssp->sda->srcu_lock_count[idx]);
> + this_cpu_inc(ssp->sda->srcu_lock_count[idx].counter);
> smp_mb(); /* B */ /* Avoid leaking the critical section. */
> return idx;
> }

Is there any particular reason that you are directly modifying @counter
instead of raw_cpu_ptr()+atomic_long_inc() that do you in
__srcu_read_lock_nmisafe() of patch 2?

> @@ -650,7 +650,7 @@ EXPORT_SYMBOL_GPL(__srcu_read_lock);
> void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
> {
> smp_mb(); /* C */ /* Avoid leaking the critical section. */
> - this_cpu_inc(ssp->sda->srcu_unlock_count[idx]);
> + this_cpu_inc(ssp->sda->srcu_unlock_count[idx].counter);
> }
> EXPORT_SYMBOL_GPL(__srcu_read_unlock);

Ditto.

> @@ -1687,8 +1687,8 @@ void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf)
> struct srcu_data *sdp;
>
> sdp = per_cpu_ptr(ssp->sda, cpu);
> - u0 = data_race(sdp->srcu_unlock_count[!idx]);
> - u1 = data_race(sdp->srcu_unlock_count[idx]);
> + u0 = data_race(sdp->srcu_unlock_count[!idx].counter);
> + u1 = data_race(sdp->srcu_unlock_count[idx].counter);
>
> /*
> * Make sure that a lock is always counted if the corresponding

And instead of atomic_long_read().

> @@ -1696,8 +1696,8 @@ void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf)
> */
> smp_rmb();
>
> - l0 = data_race(sdp->srcu_lock_count[!idx]);
> - l1 = data_race(sdp->srcu_lock_count[idx]);
> + l0 = data_race(sdp->srcu_lock_count[!idx].counter);
> + l1 = data_race(sdp->srcu_lock_count[idx].counter);
>
> c0 = l0 - u0;
> c1 = l1 - u1;

Ditto.

John Ogness