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

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

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.

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
>