Re: [PATCH] generic dynamic per cpu refcounting

[Oleg Nesterov <oleg@redhat.com>]

On 01/25, Oleg Nesterov wrote:
>
> > +void __percpu_ref_get(struct percpu_ref *ref, bool alloc)
> > +{
> > +	unsigned long pcpu_count;
> > +	uint64_t v;
> > +
> > +	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
> > +
> > +	if (REF_STATUS(pcpu_count) == PCPU_REF_PTR) {
> > +		/* for rcu - we're not using rcu_dereference() */
> > +		smp_read_barrier_depends();
> > +		__this_cpu_inc(*((unsigned __percpu *) pcpu_count));
>
> The comment looks confusing a bit... smp_read_barrier_depends() is not
> for rcu, we obviously need it to access (unsigned __percpu *) pcpu_count.
> But yes, since we didn't use rcu_dereference() we have to add it by hand.

Hmm. Otoh, arch/alpha uses asm-generic/percpu.h so in theory we need
smp_read_barrier_depends() after __this_cpu_generic_to_op() calculates the
"real" pointer, __this_cpu_ptr() ?

Just curious...

Oleg.