Re: [PATCH] cmpxchg: allow const-qualified old value in cmpxchg()

Peter Zijlstra
2026-04-02T10:47:42Z

On Thu, Apr 02, 2026 at 02:56:01PM +0800, Hangbin Liu wrote:
> The old value passed to cmpxchg() is semantically read-only: it is
> only loaded into a register as a comparand and is never written back.
> However, the macro currently assigns it implicitly to a local variable
> of type __typeof__(*(ptr)), which triggers -Werror=discarded-qualifiers
> when old is a const-qualified pointer and ptr points to a non-const type.
>
> To avoid this, let's add an explicit cast to __typeof__(*(ptr)) for the
> old local variable in the cmpxchg macros. This explicit cast suppresses
> the -Wdiscarded-qualifiers diagnostic.
>
> The new value is intentionally left without a cast: new will be stored
> into *ptr, so silently accepting a const-qualified new would allow
> callers to store a pointer-to-const into a non-const location without
> any compiler warning.
>
> Suggested-by: Jakub Kicinski <kuba@kernel.org>
> Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
> ---

Where is this a problem? I've never seen this. Also, new code should
probably use try_cmpxchg() anyway.