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" <[email protected]>
> > Signed-off-by: Alexander Gordeev <[email protected]>
>
> 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
[email protected]
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