Re: [PATCH tip/core/rcu 10/15] rcu: Protect rcu_node accesses during CPU stall warnings

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Sep 06, 2012 at 04:51:18PM +0200, Peter Zijlstra wrote:
> On Thu, 2012-08-30 at 11:56 -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@linux.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@linux.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);
> >  	}
> 
> You now cover printk() and all that that can call with a RCU lock.. is
> this a good thing?

Not particularly good.  However, this happens only if something manages to
block a grace period for 60 seconds, so it should not happen in normal
circumstances.  If that happens, holding the lock allows consistent
state to be printed, which can be helpful in tracking down the bug,
be it in RCU or elsewhere.  So the disease really is worse than the cure.

							Thanx, Paul