rcu: Make RCU able to tolerate undefined CONFIG_RCU_FANOUT

This commit introduces an RCU_FANOUT C-preprocessor macro so that RCU will
build even when CONFIG_RCU_FANOUT is undefined.  The RCU_FANOUT macro is
set to the value of CONFIG_RCU_FANOUT when defined, otherwise it is set
to 32 for 32-bit systems and 64 for 64-bit systems.  This commit then
makes CONFIG_RCU_FANOUT depend on CONFIG_RCU_EXPERT, so that Kconfig
users won't be asked about CONFIG_RCU_FANOUT unless they want to be.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>

3 files changed, 20 insertions, 8 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1bc14c670641..ba3f8d59d948 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3971,7 +3971,7 @@ static void __init rcu_init_levelspread(struct rcu_state *rsp)
         if (rcu_fanout_exact) {
                 rsp->levelspread[rcu_num_lvls - 1] = rcu_fanout_leaf;
                 for (i = rcu_num_lvls - 2; i >= 0; i--)
-                        rsp->levelspread[i] = CONFIG_RCU_FANOUT;
+                        rsp->levelspread[i] = RCU_FANOUT;
         } else {
                 int ccur;
                 int cprv;
@@ -4111,7 +4111,7 @@ static void __init rcu_init_geometry(void)
         rcu_capacity[0] = 1;
         rcu_capacity[1] = rcu_fanout_leaf;
         for (i = 2; i <= MAX_RCU_LVLS; i++)
-                rcu_capacity[i] = rcu_capacity[i - 1] * CONFIG_RCU_FANOUT;
+                rcu_capacity[i] = rcu_capacity[i - 1] * RCU_FANOUT;
 
         /*
          * The boot-time rcu_fanout_leaf parameter is only permitted
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index a69d3dab2ec4..ac3020fff028 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -35,11 +35,23 @@
  * In practice, this did work well going from three levels to four.
  * Of course, your mileage may vary.
  */
+
 #define MAX_RCU_LVLS 4
+
+#ifdef CONFIG_RCU_FANOUT
+#define RCU_FANOUT CONFIG_RCU_FANOUT
+#else /* #ifdef CONFIG_RCU_FANOUT */
+# ifdef CONFIG_64BIT
+# define RCU_FANOUT 64
+# else
+# define RCU_FANOUT 32
+# endif
+#endif /* #else #ifdef CONFIG_RCU_FANOUT */
+
 #define RCU_FANOUT_1          (CONFIG_RCU_FANOUT_LEAF)
-#define RCU_FANOUT_2          (RCU_FANOUT_1 * CONFIG_RCU_FANOUT)
-#define RCU_FANOUT_3          (RCU_FANOUT_2 * CONFIG_RCU_FANOUT)
-#define RCU_FANOUT_4          (RCU_FANOUT_3 * CONFIG_RCU_FANOUT)
+#define RCU_FANOUT_2          (RCU_FANOUT_1 * RCU_FANOUT)
+#define RCU_FANOUT_3          (RCU_FANOUT_2 * RCU_FANOUT)
+#define RCU_FANOUT_4          (RCU_FANOUT_3 * RCU_FANOUT)
 
 #if NR_CPUS <= RCU_FANOUT_1
 #  define RCU_NUM_LVLS        1
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index eb460ec747ef..d7e505970f24 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -60,10 +60,10 @@ static void __init rcu_bootup_announce_oddness(void)
 {
         if (IS_ENABLED(CONFIG_RCU_TRACE))
                 pr_info("\tRCU debugfs-based tracing is enabled.\n");
-        if ((IS_ENABLED(CONFIG_64BIT) && CONFIG_RCU_FANOUT != 64) ||
-            (!IS_ENABLED(CONFIG_64BIT) && CONFIG_RCU_FANOUT != 32))
+        if ((IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 64) ||
+            (!IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 32))
                 pr_info("\tCONFIG_RCU_FANOUT set to non-default value of %d\n",
-                       CONFIG_RCU_FANOUT);
+                       RCU_FANOUT);
         if (rcu_fanout_exact)
                 pr_info("\tHierarchical RCU autobalancing is disabled.\n");
         if (IS_ENABLED(CONFIG_RCU_FAST_NO_HZ))