Re: [RFC PATCH] rcu: using ACCESS_ONCE() to observe the jiffies_stall/rnp->qsmaskvalue

[DDD]

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.  :-)

Thanks,
Dongdong

> In any case, thank you for taking a look at RCU!
>
> 							Thanx, Paul
>
> > Signed-off-by: Dongdong Deng <dongdong.deng@xxxxxxxxxxxxx>
> > CC: Dipankar Sarma <dipankar@xxxxxxxxxx>
> > CC: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > ---
> >  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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