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

[Marco Elver]

On Tue, 8 Sept 2026 at 00:13, Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx>
> > Link: https://lore.kernel.org/all/20260903072759.GA1750084@ax162/
> > Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
>
> Thanks, this fixes that warning for me across all my builds.
>
> Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx> # build

Thanks! I think this is one of those orphaned files - which tree can
this go through? Can it go through your tree, unless Jan already
picked it up with the other fix?