Re: [PATCH] percpu-refcount: relax limit on percpu_ref_reinit()

Tejun Heo
2018-09-10T16:49:28Z

Hello, Ming.

On Sun, Sep 09, 2018 at 08:58:24PM +0800, Ming Lei wrote:
> @@ -196,15 +197,6 @@ static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
>
> atomic_long_add(PERCPU_COUNT_BIAS, &ref->count);
>
> - /*
> - * Restore per-cpu operation. smp_store_release() is paired
> - * with READ_ONCE() in __ref_is_percpu() and guarantees that the
> - * zeroing is visible to all percpu accesses which can see the
> - * following __PERCPU_REF_ATOMIC clearing.
> - */

So, while the location of percpu counter resetting moved, the pairing
of store_release and READ_ONCE is still required to ensure that the
clearing is visible before the switching to percpu mode becomes
effective. Can you please rephrase and keep the above comment?

> - for_each_possible_cpu(cpu)
> - *per_cpu_ptr(percpu_count, cpu) = 0;
> -
> smp_store_release(&ref->percpu_count_ptr,
> ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
> }
...
> @@ -357,10 +349,11 @@ EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
> void percpu_ref_reinit(struct percpu_ref *ref)
> {
> unsigned long flags;
> + unsigned long __percpu *percpu_count;
>
> spin_lock_irqsave(&percpu_ref_switch_lock, flags);
>
> - WARN_ON_ONCE(!percpu_ref_is_zero(ref));
> + WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count));

Can you elaborate this part? This doesn't seem required for the
described change. Why is it part of the patch?

Thanks.

--
tejun