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" <paul...@linux.vnet.ibm.com>
> Signed-off-by: Alexander Gordeev <agord...@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.
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]) {
> + 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
>
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