Re: [PATCH 1/9] rcu: Panic if RCU tree can not accommodate all CPUs

[Alexander Gordeev <agordeev@redhat.com>]

On Sat, Mar 07, 2015 at 09:42:34AM -0800, Paul E. McKenney wrote:
> On Sat, Mar 07, 2015 at 06:03:36PM +0100, Alexander Gordeev wrote:
> > Currently a condition when RCU tree is unable to accommodate
> > the configured number of CPUs is not permitted and causes
> > a fall back to compile-time values. However, the code has no
> > means to exceed the RCU tree capacity neither at compile-time
> > nor in run-time. Therefore, if the condition is met in run-
> > time then it indicates a serios problem elsewhere and should
> > be handled with a panic.
> > 
> > Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
> 
> The place to put a check like this is in the code that calculates
> nr_cpu_ids.  And at least some (perhaps all) are set up so that nr_cpu_ids
> cannot exceed NR_CPUS, which would render this check redundant.

The emphasis here the existing check (... n > rcu_capacity[MAX_RCU_LVLS])
(below as [1]) should not cause the fall back to compiled-time values.
It either must panic or, as you say - redundant.

> So I have to say "no" to this one.
> 
> 							Thanx, Paul
> 
> > ---
> >  kernel/rcu/tree.c | 15 +++++++++------
> >  1 file changed, 9 insertions(+), 6 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 48d640c..7588c7f 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3889,16 +3889,19 @@ static void __init rcu_init_geometry(void)
> >  		rcu_capacity[i] = rcu_capacity[i - 1] * CONFIG_RCU_FANOUT;
> > 
> >  	/*
> > +	 * The tree must be able to accommodate the configured number of CPUs.
> > +	 * If this limit is exceeded than we have a serious problem elsewhere.
> > +	 *
> >  	 * The boot-time rcu_fanout_leaf parameter is only permitted
> >  	 * to increase the leaf-level fanout, not decrease it.  Of course,
> >  	 * the leaf-level fanout cannot exceed the number of bits in
> > -	 * the rcu_node masks.  Finally, the tree must be able to accommodate
> > -	 * the configured number of CPUs.  Complain and fall back to the
> > -	 * compile-time values if these limits are exceeded.
> > +	 * the rcu_node masks.  Complain and fall back to the compile-
> > +	 * time values if these limits are exceeded.
> >  	 */
> > -	if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
> > -	    rcu_fanout_leaf > sizeof(unsigned long) * 8 ||
> > -	    n > rcu_capacity[MAX_RCU_LVLS]) {

            [1]

> > +	if (n > rcu_capacity[MAX_RCU_LVLS])
> > +		panic("rcu_init_geometry: rcu_capacity[] is too small");
> > +	else if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
> > +		 rcu_fanout_leaf > sizeof(unsigned long) * 8) {
> >  		WARN_ON(1);
> >  		return;
> >  	}
> > -- 
> > 1.8.3.1
> > 
> 

-- 
Regards,
Alexander Gordeev
agordeev@redhat.com