Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context

Peter Zijlstra
2017-06-06T17:23:42Z

On Mon, Jun 05, 2017 at 03:09:50PM -0700, Paul E. McKenney wrote:
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 3ae8474557df..157654fa436a 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -357,7 +357,7 @@ EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
>
> /*
> * Counts the new reader in the appropriate per-CPU element of the
> - * srcu_struct. Must be called from process context.
> + * srcu_struct.
> * Returns an index that must be passed to the matching srcu_read_unlock().
> */
> int __srcu_read_lock(struct srcu_struct *sp)
> @@ -365,7 +365,7 @@ int __srcu_read_lock(struct srcu_struct *sp)
> int idx;
>
> idx = READ_ONCE(sp->srcu_idx) & 0x1;
> - __this_cpu_inc(sp->sda->srcu_lock_count[idx]);
> + this_cpu_inc(sp->sda->srcu_lock_count[idx]);
> smp_mb(); /* B */ /* Avoid leaking the critical section. */
> return idx;
> }

So again, the change is to make this an IRQ safe operation, however if
we have this balance requirement, the IRQ will not visibly change the
value and load-store should be good again, no?

Or am I missing some other detail with this implementation?