linux-kernel - Re: [RFC PATCH] rcu: using ACCESS_ONCE() to observe the jiffies

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Message-ID: <20101007004339.GT2433@linux.vnet.ibm.com>
Date:	Wed, 6 Oct 2010 17:43:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	DDD <dongdong.deng@...driver.com>
Cc:	dipankar@...ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] rcu: using ACCESS_ONCE() to observe the
 jiffies_stall/rnp->qsmask value

On Wed, Sep 29, 2010 at 02:46:18PM +0800, DDD wrote:
> Paul E. McKenney wrote:
> >On Tue, Sep 28, 2010 at 04:32:43PM +0800, Dongdong Deng wrote:
> >>Using ACCESS_ONCE() to observe the jiffies_stall/rnp->qsmask value
> >>due to the caller didn't hold the root_rcu/rnp node's lock.
> >
> >Given that the quantities are only used once, ACCESS_ONCE() is strictly
> >speaking not necessary.  I am tempted to queue your patch regardless
> >for the documentation benefits.
> >
> >Or is this patch necessary to fix a failure that you have been seeing?
> 
> Hello Pual,
> 
> No, I didn't see a failure about it.
> I got this patch from code review.  :-)

Thank you for the info, patch queued.

							Thanx, Paul

> Thanks,
> Dongdong
> 
> >In any case, thank you for taking a look at RCU!
> >
> >							Thanx, Paul
> >
> >>Signed-off-by: Dongdong Deng <dongdong.deng@...driver.com>
> >>CC: Dipankar Sarma <dipankar@...ibm.com>
> >>CC: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> >>---
> >> kernel/rcutree.c |    4 ++--
> >> 1 files changed, 2 insertions(+), 2 deletions(-)
> >>
> >>diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> >>index d5bc439..07a7063 100644
> >>--- a/kernel/rcutree.c
> >>+++ b/kernel/rcutree.c
> >>@@ -532,9 +532,9 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
> >>
> >> 	if (rcu_cpu_stall_panicking)
> >> 		return;
> >>-	delta = jiffies - rsp->jiffies_stall;
> >>+	delta = jiffies - ACCESS_ONCE(rsp->jiffies_stall);
> >> 	rnp = rdp->mynode;
> >>-	if ((rnp->qsmask & rdp->grpmask) && delta >= 0) {
> >>+	if ((ACCESS_ONCE(rnp->qsmask) & rdp->grpmask) && delta >= 0) {
> >>
> >> 		/* We haven't checked in, so go dump stack. */
> >> 		print_cpu_stall(rsp);
> >>-- 
> >>1.6.0.4
> >>
> >
> 
> 
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