fprobes: Add a comment why fprobe_kprobe_handler exits if kprobe is running

Add a comment the reason why fprobe_kprobe_handler() exits if any other
kprobe is running.

Link: https://lore.kernel.org/all/168874788299.159442.2485957441413653858.stgit@devnote2/Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Link: https://lore.kernel.org/all/20230706120916.3c6abf15@gandalf.local.home/Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/fprobe.c

@@ -100,6 +100,12 @@ static void fprobe_kprobe_handler(unsigned long ip, unsigned long parent_ip,
 		return;
 	}
 
+	/*
+	 * This user handler is shared with other kprobes and is not expected to be
+	 * called recursively. So if any other kprobe handler is running, this will
+	 * exit as kprobe does. See the section 'Share the callbacks with kprobes'
+	 * in Documentation/trace/fprobe.rst for more information.
+	 */
 	if (unlikely(kprobe_running())) {
 		fp->nmissed++;
 		goto recursion_unlock;