rcu: Panic if RCU tree can not accommodate all CPUs

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>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 17 insertions, 12 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1c58cbd03922..fe8d92987dfa 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4107,6 +4107,19 @@ 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.
+         */
+        if (rcu_fanout_leaf < RCU_FANOUT_LEAF ||
+            rcu_fanout_leaf > sizeof(unsigned long) * 8) {
+                WARN_ON(1);
+                return;
+        }
+
+        /*
          * Compute number of nodes that can be handled an rcu_node tree
          * with the given number of levels.  Setting rcu_capacity[0] makes
          * some of the arithmetic easier.
@@ -4117,19 +4130,11 @@ static void __init rcu_init_geometry(void)
                 rcu_capacity[i] = rcu_capacity[i - 1] * RCU_FANOUT;
 
         /*
-         * 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 tree must be able to accommodate the configured number of CPUs.
+         * If this limit is exceeded than we have a serious problem elsewhere.
          */
-        if (rcu_fanout_leaf < RCU_FANOUT_LEAF ||
-            rcu_fanout_leaf > sizeof(unsigned long) * 8 ||
-            n > rcu_capacity[MAX_RCU_LVLS]) {
-                WARN_ON(1);
-                return;
-        }
+        if (n > rcu_capacity[MAX_RCU_LVLS])
+                panic("rcu_init_geometry: rcu_capacity[] is too small");
 
         /* Calculate the number of rcu_nodes at each level of the tree. */
         for (i = 1; i <= MAX_RCU_LVLS; i++)