Messages in this thread | | | Date | Tue, 8 Jul 2014 14:40:11 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 06/17] rcu: Eliminate read-modify-write ACCESS_ONCE() calls |
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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@linux.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@gmail.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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