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once_lite: Simplify condition handling and fix context analysis

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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@kernel.org>
Link: https://lore.kernel.org/all/20260903072759.GA1750084@ax162/
Signed-off-by: Marco Elver <elver@google.com>
Tested-by: Nathan Chancellor <nathan@kernel.org> # build
Link: https://patch.msgid.link/20260903101843.3462767-1-elver@google.com
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
This commit is contained in:
Marco Elver 2026-09-03 10:16:38 +00:00 committed by Nathan Chancellor
parent cee9395acd
commit 0bb666d5f5
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GPG Key ID: 1D6B269171C01A96
1 changed files with 5 additions and 3 deletions
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8
include/linux/once_lite.h
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@ -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); \