[PATCH] once_lite: Simplify condition handling and fix context analysis

[Marco Elver]

When WARN_ON_ONCE() wraps a conditional lock acquisition (such as
down_write_trylock()) on architectures relying on DO_ONCE_LITE_IF()
(e.g. arm), Clang's context analysis (Thread Safety Analysis) failed
with a false positive:

  fs/ext2/xattr.c:825:6: error: rw_semaphore 'EXT2_I().xattr_sem' is not held on every path through here [-Werror,-Wthread-safety-analysis]
    825 |         if (WARN_ON_ONCE(!down_write_trylock(&EXT2_I(inode)->xattr_sem)))
        |             ^

This happens because DO_ONCE_LITE_IF() branches on __ONCE_LITE_IF()'s
return value (__ret_once), creating an intermediate branch merge point
where the lock may or may not be held depending on whether the once-flag
(__already_done) was already set. Because the merge branch condition is
__ret_once rather than the trylock predicate (__ret_do_once), Clang
cannot reconcile the lockset at the branch merge points.

Fix it by refactoring __ONCE_LITE_IF() into an unconditional
__ONCE_LITE() primitive and redefining __ONCE_LITE_IF(condition) as:

  (unlikely(condition) && __ONCE_LITE())

This simplifies the implementation, short-circuits evaluation so that
__ONCE_LITE() is not called when the condition is false, and ensures
that DO_ONCE_LITE_IF() only enters __ONCE_LITE() when __ret_do_once is
true.

Reported-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Link: https://lore.kernel.org/all/20260903072759.GA1750084@ax162/
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
 include/linux/once_lite.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/linux/once_lite.h b/include/linux/once_lite.h
index 236592c4eeb1..5e2b67039aaf 100644
--- a/include/linux/once_lite.h
+++ b/include/linux/once_lite.h
@@ -10,19 +10,21 @@
 #define DO_ONCE_LITE(func, ...)						\
 	DO_ONCE_LITE_IF(true, func, ##__VA_ARGS__)
 
-#define __ONCE_LITE_IF(condition)					\
+#define __ONCE_LITE()							\
 	({								\
 		static bool __section(".data..once") __already_done;	\
-		bool __ret_cond = !!(condition);			\
 		bool __ret_once = false;				\
 									\
-		if (unlikely(__ret_cond) && unlikely(!__already_done)) {\
+		if (unlikely(!__already_done)) {			\
 			__already_done = true;				\
 			__ret_once = true;				\
 		}							\
 		unlikely(__ret_once);					\
 	})
 
+#define __ONCE_LITE_IF(condition)					\
+	(unlikely(condition) && __ONCE_LITE())
+
 #define DO_ONCE_LITE_IF(condition, func, ...)				\
 	({								\
 		bool __ret_do_once = !!(condition);			\
-- 
2.55.0.970.g62bdec98f9-goog