linux-kernel - Re: [PATCH tip/core/rcu 06/17] rcu: Eliminate read-modify-write ACCESS

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Date:	Tue, 8 Jul 2014 14:40:11 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, mingo@...nel.org,
	laijs@...fujitsu.com, Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Triplett <josh@...htriplett.org>, niv@...ibm.com,
	tglx@...utronix.de, Peter Zijlstra <peterz@...radead.org>,
	rostedt@...dmis.org, dhowells@...hat.com,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH tip/core/rcu 06/17] rcu: Eliminate read-modify-write
 ACCESS_ONCE() calls

On Tue, Jul 08, 2014 at 04:43:37PM -0400, Pranith Kumar wrote:
> On Tue, Jul 8, 2014 at 4:35 PM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> >
> > Good catch, I clearly didn't include enough patterns in my search.
> >
> > But please see below.  And please rebase onto branch rcu/dev in
> > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git,
> > as this patch set does not apply.
> 
> OK, I will resend the patch. One question below:
> 
> >
> >                                                         Thanx, Paul
> >
> >> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
> >> ---
> >> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> >> index dac6d20..f500395 100644
> >> --- a/kernel/rcu/tree.c
> >> +++ b/kernel/rcu/tree.c
> >> @@ -1700,7 +1700,7 @@ static int rcu_gp_fqs(struct rcu_state *rsp, int
> >> fqs_state_in)
> >>         if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
> >>                 raw_spin_lock_irq(&rnp->lock);
> >>                 smp_mb__after_unlock_lock();
> >> -               ACCESS_ONCE(rsp->gp_flags) &= ~RCU_GP_FLAG_FQS;
> >> +               ACCESS_ONCE(rsp->gp_flags) = rsp->gp_flags & ~RCU_GP_FLAG_FQS;
> >
> > Here we need ACCESS_ONCE() around both instances of rsp->gp_flags.
> 
> I see that all accesses of gp_flags are wrapped with ACCESS_ONCE(). Is
> there any reason why we can't declare it as 'volatile' and not use
> ACCESS_ONCE everywhere?

The explicit ACCESS_ONCE() serves as a good documentation aid.

							Thanx, Paul

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