Re: [PATCH v2 1/6] ARM: cmpxchg: always inline __arch_xchg() and __cmpxchg()

[Bradley Morgan]

On 8 October 2026 06:37:41 BST, Karl Mehltretter <kmehltretter@xxxxxxxxx>
wrote:
>__arch_xchg(), __cmpxchg() and __cmpxchg_local() end in a default case
>that calls an undefined function. That turns an unsupported size into a
>link error. It relies on the functions being inlined, so that the
>compiler can drop the default case for a constant size.
>
>They are only marked inline. gcc 8.1.0 with CC_OPTIMIZE_FOR_SIZE does
>not inline them in rust/helpers/helpers.c, which calls xchg() and
>cmpxchg() from many small helpers. The out-of-line copies keep the
>default case and the link fails.
>
>  helpers.c:(.text+0x424): undefined reference to `__bad_xchg'
>  helpers.c:(.text+0x4ac): undefined reference to `__bad_cmpxchg'
>
>Seen with bcm2835_defconfig and CONFIG_RUST=y on v7.3-rc1. The same
>config links with CONFIG_RUST=n. It also links with gcc 15.2.0 and
>with clang.
>
>v6.19 links. Commit ab717dd98bee ("rust: helpers: Add i8/i16 atomic
>xchg_acquire helpers") in v7.0 triggers __bad_xchg. Commit
>ac8f06ade38a ("rust: sync: atomic: Add Atomic<*{mut,const} T> support")
>in v7.1 also triggers __bad_cmpxchg.
>
>Mark the functions __always_inline.
>
>Fixes: ab717dd98bee ("rust: helpers: Add i8/i16 atomic xchg_acquire helpers")
>Fixes: ac8f06ade38a ("rust: sync: atomic: Add Atomic<*{mut,const} T> support")
>Cc: stable@xxxxxxxxxxxxxxx
>Assisted-by: LLM

Looks fine cmpxchg ended 

Reviewed-by: Bradley Morgan <brads@xxxxxxxxxxxxxx>


>Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
>---
> arch/arm/include/asm/cmpxchg.h | 12 ++++++------
> 1 file changed, 6 insertions(+), 6 deletions(-)
>
>diff --git a/arch/arm/include/asm/cmpxchg.h b/arch/arm/include/asm/cmpxchg.h
>index 9beb64d30586..0ce5225af442 100644
>--- a/arch/arm/include/asm/cmpxchg.h
>+++ b/arch/arm/include/asm/cmpxchg.h
>@@ -26,7 +26,7 @@
> #define swp_is_buggy
> #endif
> 
>-static inline unsigned long
>+static __always_inline unsigned long
> __arch_xchg(unsigned long x, volatile void *ptr, int size)
> {
> 	extern void __bad_xchg(volatile void *, int);
>@@ -155,8 +155,8 @@ extern void __bad_cmpxchg(volatile void *ptr, int size);
>  * cmpxchg only support 32-bits operands on ARMv6.
>  */
> 
>-static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
>-				      unsigned long new, int size)
>+static __always_inline unsigned long
>+__cmpxchg(volatile void *ptr, unsigned long old, unsigned long new, int size)
> {
> 	unsigned long oldval, res;
> 
>@@ -220,9 +220,9 @@ static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
> 				      sizeof(*(ptr)));			\
> })
> 
>-static inline unsigned long __cmpxchg_local(volatile void *ptr,
>-					    unsigned long old,
>-					    unsigned long new, int size)
>+static __always_inline unsigned long
>+__cmpxchg_local(volatile void *ptr, unsigned long old, unsigned long new,
>+		int size)
> {
> 	unsigned long ret;
> 
>

--- Thanks!
"I'm not a very positive person" - Linus torvalds