On Fri, Nov 02, 2012 at 08:19:04PM +0000, Christoph Lameter wrote:
> 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 <davids...@tencent.com>
> > >
> > > Signed-off-by: Shan Wei <davids...@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.
OK, I do understand why it happens to work. My question is instead why
it is considered a good idea. After all, it is the ->rda field that is
marked __percpu, not the ->mynode field. So in the interest of
mechanical checking and general readability, it seems to me that it
would be way better to apply __this_cpu_read() to rsp->rda rather than
to rsp->rda->mynode.
Thanx, Paul
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
