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>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1c58cbd03922ac4323c273853c1e47e114c40bdd..fe8d92987dfa9941937d09c3999cb5c7f9af3af4 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4106,6 +4106,19 @@ static void __init rcu_init_geometry(void)
        pr_info("RCU: Adjusting geometry for rcu_fanout_leaf=%d, nr_cpu_ids=%d\n",
                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
@@ -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++)