rcu: Eliminate panic when silly boot-time fanout specified

This commit loosens rcutree.rcu_fanout_leaf range checks
and replaces a panic() with a fallback to compile-time values.
This fallback is accompanied by a WARN_ON(), and both occur when the
rcutree.rcu_fanout_leaf value is too small to accommodate the number of
CPUs.  For example, given the current four-level limit for the rcu_node
tree, a system with more than 16 CPUs built with CONFIG_FANOUT=2 must
have rcutree.rcu_fanout_leaf larger than 2.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

1 files changed, 11 insertions, 9 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 93c0f23c3e45..713eb92314b4 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4230,13 +4230,12 @@ static void __init rcu_init_geometry(void)
                 rcu_fanout_leaf, nr_cpu_ids);
 
         /*
-         * 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.  Complain and fall back to the compile-
-         * time values if these limits are exceeded.
+         * The boot-time rcu_fanout_leaf parameter must be at least two
+         * and cannot exceed the number of bits in the rcu_node masks.
+         * Complain and fall back to the compile-time values if this
+         * limit is exceeded.
          */
-        if (rcu_fanout_leaf < RCU_FANOUT_LEAF ||
+        if (rcu_fanout_leaf < 2 ||
             rcu_fanout_leaf > sizeof(unsigned long) * 8) {
                 rcu_fanout_leaf = RCU_FANOUT_LEAF;
                 WARN_ON(1);
@@ -4253,10 +4252,13 @@ static void __init rcu_init_geometry(void)
 
         /*
          * The tree must be able to accommodate the configured number of CPUs.
-         * If this limit is exceeded than we have a serious problem elsewhere.
+         * If this limit is exceeded, fall back to the compile-time values.
          */
-        if (nr_cpu_ids > rcu_capacity[RCU_NUM_LVLS - 1])
-                panic("rcu_init_geometry: rcu_capacity[] is too small");
+        if (nr_cpu_ids > rcu_capacity[RCU_NUM_LVLS - 1]) {
+                rcu_fanout_leaf = RCU_FANOUT_LEAF;
+                WARN_ON(1);
+                return;
+        }
 
         /* Calculate the number of levels in the tree. */
         for (i = 0; nr_cpu_ids > rcu_capacity[i]; i++) {