linux-kernel - Re: [PATCH tip/core/rcu 10/15] rcu: Protect rcu

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Date:	Tue, 4 Sep 2012 15:51:53 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 10/15] rcu: Protect rcu_node accesses during
 CPU stall warnings

On Fri, Aug 31, 2012 at 11:23:02AM -0700, Josh Triplett wrote:
> On Thu, Aug 30, 2012 at 11:56:23AM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > 
> > The print_other_cpu_stall() function accesses a number of rcu_node
> > fields without protection from the ->lock.  In theory, this is not
> > a problem because the fields accessed are all integers, but in
> > practice the compiler can get nasty.  Therefore, the commit extends
> > the existing critical section to cover the entire loop body.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> >  kernel/rcutree.c |    6 ++++--
> >  1 files changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 9f44749..fbe43b0 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -746,14 +746,16 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
> >  	rcu_for_each_leaf_node(rsp, rnp) {
> >  		raw_spin_lock_irqsave(&rnp->lock, flags);
> >  		ndetected += rcu_print_task_stall(rnp);
> > -		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> > -		if (rnp->qsmask == 0)
> > +		if (rnp->qsmask == 0) {
> > +			raw_spin_unlock_irqrestore(&rnp->lock, flags);
> >  			continue;
> > +		}
> >  		for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
> >  			if (rnp->qsmask & (1UL << cpu)) {
> >  				print_cpu_stall_info(rsp, rnp->grplo + cpu);
> >  				ndetected++;
> >  			}
> > +		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> >  	}
> 
> Now that you've extended the lock over the rest of the loop body, I
> think this would look much clearer if written without the continue and
> duplicate lock release:
> 
> 		...
> 		if (rnp->qsmask != 0)
> 			for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
> 				....
> 		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> 	}

And my Hollerith experience strikes again!  ;-)

Though this one seems more worthwhile, so I am making the change, conflicts
permitting.

							Thanx, Paul

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