linux-kernel - Re: [PATCH] generic dynamic per cpu refcounting

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Message-ID: <20130125182924.GA17514@redhat.com>
Date:	Fri, 25 Jan 2013 19:29:24 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	tj@...nel.org, srivatsa.bhat@...ux.vnet.ibm.com,
	rusty@...tcorp.com.au, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] generic dynamic per cpu refcounting

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.

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