ftrace: Add comments to ftrace_hash_move() and friends

Describe what ftrace_hash_move() does and add some more comments to some
other functions to make it easier to understand.

Link: https://lore.kernel.org/linux-trace-kernel/20240605180409.179520305@goodmis.org

Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -169,6 +169,7 @@ static inline void ftrace_ops_init(struct ftrace_ops *ops)
 #endif
 }
 
+/* Call this function for when a callback filters on set_ftrace_pid */
 static void ftrace_pid_func(unsigned long ip, unsigned long parent_ip,
 			    struct ftrace_ops *op, struct ftrace_regs *fregs)
 {
@@ -1318,7 +1319,7 @@ static struct ftrace_hash *alloc_ftrace_hash(int size_bits)
 	return hash;
 }
 
-
+/* Used to save filters on functions for modules not loaded yet */
 static int ftrace_add_mod(struct trace_array *tr,
 			  const char *func, const char *module,
 			  int enable)
@@ -1430,6 +1431,7 @@ static struct ftrace_hash *__move_hash(struct ftrace_hash *src, int size)
 	return new_hash;
 }
 
+/* Move the @src entries to a newly allocated hash */
 static struct ftrace_hash *
 __ftrace_hash_move(struct ftrace_hash *src)
 {
@@ -1444,6 +1446,26 @@ __ftrace_hash_move(struct ftrace_hash *src)
 	return __move_hash(src, size);
 }
 
+/**
+ * ftrace_hash_move - move a new hash to a filter and do updates
+ * @ops: The ops with the hash that @dst points to
+ * @enable: True if for the filter hash, false for the notrace hash
+ * @dst: Points to the @ops hash that should be updated
+ * @src: The hash to update @dst with
+ *
+ * This is called when an ftrace_ops hash is being updated and the
+ * the kernel needs to reflect this. Note, this only updates the kernel
+ * function callbacks if the @ops is enabled (not to be confused with
+ * @enable above). If the @ops is enabled, its hash determines what
+ * callbacks get called. This function gets called when the @ops hash
+ * is updated and it requires new callbacks.
+ *
+ * On success the elements of @src is moved to @dst, and @dst is updated
+ * properly, as well as the functions determined by the @ops hashes
+ * are now calling the @ops callback function.
+ *
+ * Regardless of return type, @src should be freed with free_ftrace_hash().
+ */
 static int
 ftrace_hash_move(struct ftrace_ops *ops, int enable,
 		 struct ftrace_hash **dst, struct ftrace_hash *src)