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

[Nathan Chancellor <nathan@kernel.org>]

On Thu, Sep 03, 2026 at 10:16:38AM +0000, Marco Elver wrote:
> 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>

Thanks, this fixes that warning for me across all my builds.

Tested-by: Nathan Chancellor <nathan@kernel.org> # build

-- 
Cheers,
Nathan