On Thu, Dec 18, 2025 at 10:19:42AM -0500, Steven Rostedt wrote:

On Mon, 15 Dec 2025 22:13:59 +0100
Jiri Olsa [off-list ref] wrote:

quoted

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 48dc0de5f2ce..95a38fb18ed7 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c

@@ -6489,6 +6489,78 @@ int update_ftrace_direct_del(struct ftrace_ops *ops, struct ftrace_hash *hash)
 	return err;
 }
 

Kerneldoc needed.

will add

quoted

+int update_ftrace_direct_mod(struct ftrace_ops *ops, struct ftrace_hash *hash, bool do_direct_lock)
+{
+	struct ftrace_func_entry *entry, *tmp;
+	static struct ftrace_ops tmp_ops = {
+		.func		= ftrace_stub,
+		.flags		= FTRACE_OPS_FL_STUB,
+	};
+	struct ftrace_hash *orig_hash;
+	unsigned long size, i;
+	int err = -EINVAL;
+
+	if (!hash_count(hash))
+		return -EINVAL;
+	if (check_direct_multi(ops))
+		return -EINVAL;
+	if (!(ops->flags & FTRACE_OPS_FL_ENABLED))
+		return -EINVAL;
+	if (direct_functions == EMPTY_HASH)
+		return -EINVAL;
+
+	if (do_direct_lock)
+		mutex_lock(&direct_mutex);

This optional taking of the direct_mutex lock needs some serious rationale
and documentation.

it mirrors the use of modify_ftrace_direct/modify_ftrace_direct_nolock
when we do trampoline update from within ftrace_ops->ops_func callback

I'll add comments with more details

quoted

+
+	orig_hash = ops->func_hash ? ops->func_hash->filter_hash : NULL;
+	if (!orig_hash)
+		goto unlock;
+
+	/* Enable the tmp_ops to have the same functions as the direct ops */
+	ftrace_ops_init(&tmp_ops);
+	tmp_ops.func_hash = ops->func_hash;
+
+	err = register_ftrace_function_nolock(&tmp_ops);
+	if (err)
+		goto unlock;
+
+	/*
+	 * Call __ftrace_hash_update_ipmodify() here, so that we can call
+	 * ops->ops_func for the ops. This is needed because the above
+	 * register_ftrace_function_nolock() worked on tmp_ops.
+	 */
+	err = __ftrace_hash_update_ipmodify(ops, orig_hash, orig_hash, true);
+	if (err)
+		goto out;
+
+	/*
+	 * Now the ftrace_ops_list_func() is called to do the direct callers.
+	 * We can safely change the direct functions attached to each entry.
+	 */
+	mutex_lock(&ftrace_lock);

I'm going to need some time staring at this code. It looks like it may be
relying on some internals here.

ok, thanks

jirka