linux-kernel - Re: [PATCH v2 6/9] rcu: use __this_cpu_read helper instead of per_cpu_ptr(p, raw_smp_processor

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Date:	Sat, 3 Nov 2012 02:19:09 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Shan Wei <shanwei88@...il.com>, dipankar@...ibm.com,
	Kernel-Maillist <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 6/9] rcu: use __this_cpu_read helper instead of
 per_cpu_ptr(p, raw_smp_processor_id())

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 <davidshan@...cent.com>
> > >
> > > Signed-off-by: Shan Wei <davidshan@...cent.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

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