The __mt_dup() function requires callers to hold the appropriate write
lock when duplicating a maple tree. Without proper locking, concurrent
modifications during duplication could access invalid node slots.

Add a lockdep assertion to catch such API misuse during development.
This is API hardening rather than a bug fix - all in-tree callers
already follow the proper locking rules as documented above __mt_dup().

Signed-off-by: Boudewijn van der Heide <boudewijn@delta-utec.com>
---
Changes in v2:
    - Replaced runtime deadnode check with a lockdep assertion
v1:
    https://lore.kernel.org/lkml/20260103165758.74094-1-boudewijn@delta-utec.com/

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 5aa4c9500018..3b4357f16352 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -6248,6 +6248,8 @@ static inline void mas_dup_alloc(struct ma_state *mas, struct ma_state *new_mas,
 	void __rcu **new_slots;
 	unsigned long val;
 
+	lockdep_assert(mt_write_locked(mas->tree));
+
 	/* Allocate memory for child nodes. */
 	type = mte_node_type(mas->node);
 	new_slots = ma_slots(new_node, type);
--
2.47.3