[PATCH] rcu: Improve comments for RCU_FANOUT and RCU_FANOUT_LEAF

[Kunwu Chan <kunwu.chan@linux.dev>]

From: Kunwu Chan <chentao@kylinos.cn>

The original comments introduced in commit 05c5df31afd1
("rcu: Make RCU able to tolerate undefined CONFIG_RCU_FANOUT"),
contained confusing annotations.

Specifically, the #else and #endif comments did not clearly reflect
their corresponding condition blocks, hampering readability.

Fixes condition branch comments. And adds explicit explanations of
the overall purpose:
defining middle/leaf fan-out parameters, their relation to Kconfig,
and how they shape the RCU hierarchy based on CPU count.

Make the hierarchical configuration logic of the RCU easier to understand.

Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
---
 include/linux/rcu_node_tree.h | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/include/linux/rcu_node_tree.h b/include/linux/rcu_node_tree.h
index 78feb8ba7358..b03c0ce91dec 100644
--- a/include/linux/rcu_node_tree.h
+++ b/include/linux/rcu_node_tree.h
@@ -25,26 +25,34 @@
 /*
  * Define shape of hierarchy based on NR_CPUS, CONFIG_RCU_FANOUT, and
  * CONFIG_RCU_FANOUT_LEAF.
+ * - RCU_FANOUT: Controls fan-out of middle levels in the RCU hierarchy.
+ * - RCU_FANOUT_LEAF: Controls fan-out of the leaf level (directly managing CPUs).
+ *
+ * These parameters are determined by Kconfig options if configured; otherwise,
+ * they use sensible defaults based on system architecture (for RCU_FANOUT)
+ * or a fixed default (for RCU_FANOUT_LEAF).
  * In theory, it should be possible to add more levels straightforwardly.
  * In practice, this did work well going from three levels to four.
  * Of course, your mileage may vary.
  */
 
+/* Define RCU_FANOUT: middle-level fan-out parameter */
 #ifdef CONFIG_RCU_FANOUT
 #define RCU_FANOUT CONFIG_RCU_FANOUT
-#else /* #ifdef CONFIG_RCU_FANOUT */
+#else /* #ifndef CONFIG_RCU_FANOUT */
 # ifdef CONFIG_64BIT
 # define RCU_FANOUT 64
 # else
 # define RCU_FANOUT 32
 # endif
-#endif /* #else #ifdef CONFIG_RCU_FANOUT */
+#endif
 
+/* Define RCU_FANOUT_LEAF: leaf-level fan-out parameter (manages CPUs directly) */
 #ifdef CONFIG_RCU_FANOUT_LEAF
 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
-#else /* #ifdef CONFIG_RCU_FANOUT_LEAF */
+#else /* #ifndef CONFIG_RCU_FANOUT_LEAF */
 #define RCU_FANOUT_LEAF 16
-#endif /* #else #ifdef CONFIG_RCU_FANOUT_LEAF */
+#endif
 
 #define RCU_FANOUT_1	      (RCU_FANOUT_LEAF)
 #define RCU_FANOUT_2	      (RCU_FANOUT_1 * RCU_FANOUT)
-- 
2.25.1