Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754639AbaGHUoL (ORCPT ); Tue, 8 Jul 2014 16:44:11 -0400 Received: from mail-lb0-f174.google.com ([209.85.217.174]:49448 "EHLO mail-lb0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752212AbaGHUoJ (ORCPT ); Tue, 8 Jul 2014 16:44:09 -0400 MIME-Version: 1.0 In-Reply-To: <20140708203459.GU4603@linux.vnet.ibm.com> References: <20140707223756.GA7187@linux.vnet.ibm.com> <1404772701-8804-1-git-send-email-paulmck@linux.vnet.ibm.com> <1404772701-8804-6-git-send-email-paulmck@linux.vnet.ibm.com> <20140708203459.GU4603@linux.vnet.ibm.com> From: Pranith Kumar Date: Tue, 8 Jul 2014 16:43:37 -0400 Message-ID: Subject: Re: [PATCH tip/core/rcu 06/17] rcu: Eliminate read-modify-write ACCESS_ONCE() calls To: Paul McKenney Cc: LKML , mingo@kernel.org, laijs@cn.fujitsu.com, Dipankar Sarma , Andrew Morton , Mathieu Desnoyers , Josh Triplett , niv@us.ibm.com, tglx@linutronix.de, Peter Zijlstra , rostedt@goodmis.org, dhowells@redhat.com, Eric Dumazet Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 8, 2014 at 4:35 PM, Paul E. McKenney 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 >> --- >> 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? -- Pranith -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/