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SubjectRe: [PATCH v2 6/9] rcu: use __this_cpu_read helper instead of per_cpu_ptr(p, raw_smp_processor_id())
On Fri, 2 Nov 2012, Paul E. McKenney wrote:

> On Sat, Nov 03, 2012 at 12:01:47AM +0800, Shan Wei wrote:
> > From: Shan Wei <davidshan@tencent.com>
> >
> > Signed-off-by: Shan Wei <davidshan@tencent.com>
> > ---
> > kernel/rcutree.c | 2 +-
> > 1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 74df86b..441b945 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -1960,7 +1960,7 @@ static void force_quiescent_state(struct rcu_state *rsp)
> > struct rcu_node *rnp_old = NULL;
> >
> > /* Funnel through hierarchy to reduce memory contention. */
> > - rnp = per_cpu_ptr(rsp->rda, raw_smp_processor_id())->mynode;
> > + rnp = __this_cpu_read(rsp->rda->mynode);
>
> OK, I'll bite... Why this instead of:
>
> rnp = __this_cpu_read(rsp->rda)->mynode;

Because this_cpu_read fetches a data word from an address. The addres is
relocated using a segment prefix (which contains the offset of the
current per cpu area).

And the address needed here is the address of the field of mynode
within a structure that has a per cpu address.


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