From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758292Ab2FOWYw (ORCPT ); Fri, 15 Jun 2012 18:24:52 -0400 Received: from e34.co.us.ibm.com ([32.97.110.152]:48715 "EHLO e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758183Ab2FOWYv (ORCPT ); Fri, 15 Jun 2012 18:24:51 -0400 Date: Fri, 15 Jun 2012 15:24:42 -0700 From: "Paul E. McKenney" To: Josh Triplett Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, patches@linaro.org, "Paul E. McKenney" Subject: Re: [PATCH tip/core/rcu 03/14] rcu: Add ACCESS_ONCE() to ->qlen accesses Message-ID: <20120615222442.GO2389@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20120615201256.GA26120@linux.vnet.ibm.com> <1339791195-26389-1-git-send-email-paulmck@linux.vnet.ibm.com> <1339791195-26389-3-git-send-email-paulmck@linux.vnet.ibm.com> <20120615204500.GH31184@leaf> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120615204500.GH31184@leaf> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12061522-1780-0000-0000-0000067CA370 X-IBM-ISS-SpamDetectors: X-IBM-ISS-DetailInfo: BY=3.00000281; HX=3.00000190; KW=3.00000007; PH=3.00000001; SC=3.00000002; SDB=6.00148371; UDB=6.00033818; UTC=2012-06-15 22:24:50 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 15, 2012 at 01:45:00PM -0700, Josh Triplett wrote: > On Fri, Jun 15, 2012 at 01:13:04PM -0700, Paul E. McKenney wrote: > > From: "Paul E. McKenney" > > > > The _rcu_barrier() function accesses other CPUs' rcu_data structure's > > ->qlen field without benefit of locking. This commit therefore adds > > the required ACCESS_ONCE() wrappers around accesses and updates that > > need it. > > This type of restriction makes me wonder if we could add some kind of > attribute to fields like qlen to make GCC or sparse help enforce this. It is worth thinking about. Should spark some spirited discussions. ;-) > > ACCESS_ONCE() is not needed when a CPU accesses its own ->qlen, or > > in code that cannot run while _rcu_barrier() is sampling ->qlen fields. > > > > Signed-off-by: Paul E. McKenney > > Reviewed-by: Josh Triplett Thank you! Thanx, Paul > > kernel/rcutree.c | 8 ++++---- > > 1 files changed, 4 insertions(+), 4 deletions(-) > > > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > > index d938671..cdc101e 100644 > > --- a/kernel/rcutree.c > > +++ b/kernel/rcutree.c > > @@ -1349,7 +1349,7 @@ rcu_send_cbs_to_orphanage(int cpu, struct rcu_state *rsp, > > rsp->qlen += rdp->qlen; > > rdp->n_cbs_orphaned += rdp->qlen; > > rdp->qlen_lazy = 0; > > - rdp->qlen = 0; > > + ACCESS_ONCE(rdp->qlen) = 0; > > } > > > > /* > > @@ -1597,7 +1597,7 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp) > > } > > smp_mb(); /* List handling before counting for rcu_barrier(). */ > > rdp->qlen_lazy -= count_lazy; > > - rdp->qlen -= count; > > + ACCESS_ONCE(rdp->qlen) -= count; > > rdp->n_cbs_invoked += count; > > > > /* Reinstate batch limit if we have worked down the excess. */ > > @@ -1886,7 +1886,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu), > > rdp = this_cpu_ptr(rsp->rda); > > > > /* Add the callback to our list. */ > > - rdp->qlen++; > > + ACCESS_ONCE(rdp->qlen)++; > > if (lazy) > > rdp->qlen_lazy++; > > else > > @@ -2420,7 +2420,7 @@ rcu_boot_init_percpu_data(int cpu, struct rcu_state *rsp) > > rdp->grpmask = 1UL << (cpu - rdp->mynode->grplo); > > init_callback_list(rdp); > > rdp->qlen_lazy = 0; > > - rdp->qlen = 0; > > + ACCESS_ONCE(rdp->qlen) = 0; > > rdp->dynticks = &per_cpu(rcu_dynticks, cpu); > > WARN_ON_ONCE(rdp->dynticks->dynticks_nesting != DYNTICK_TASK_EXIT_IDLE); > > WARN_ON_ONCE(atomic_read(&rdp->dynticks->dynticks) != 1); > > -- > > 1.7.8 > > >