Re: [PATCH] generic dynamic per cpu refcounting

[Kent Overstreet <koverstreet@google.com>]

On Fri, Jan 25, 2013 at 07:29:24PM +0100, Oleg Nesterov wrote:
> 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...

Don't think I follow, but I don't see how... that barriers just needs to
be between the ACCESS_ONCE and touching the memory pointed to, the fact
that there's a calculation in there to get the real pointer is
irrelevant...