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

[Karl Mehltretter]

__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
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;
 
-- 
2.39.5 (Apple Git-154)