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

Hangbin Liu
2026-04-02T06:56:40Z

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>
---
This is my first time posting a patch with multi-architecture specific
changes, so I'm not sure which target repository I should use. Please
correct me if I made any mistakes.
---
arch/alpha/include/asm/cmpxchg.h | 4 ++--
arch/arc/include/asm/cmpxchg.h | 4 ++--
arch/hexagon/include/asm/cmpxchg.h | 2 +-
arch/parisc/include/asm/cmpxchg.h | 2 +-
arch/powerpc/include/asm/cmpxchg.h | 8 ++++----
arch/riscv/include/asm/cmpxchg.h | 4 ++--
arch/sh/include/asm/cmpxchg.h | 2 +-
arch/sparc/include/asm/cmpxchg_32.h | 2 +-
arch/sparc/include/asm/cmpxchg_64.h | 2 +-
arch/x86/include/asm/cmpxchg.h | 2 +-
arch/xtensa/include/asm/cmpxchg.h | 2 +-
tools/arch/x86/include/asm/cmpxchg.h | 2 +-
12 files changed, 18 insertions(+), 18 deletions(-)

diff --git a/arch/alpha/include/asm/cmpxchg.h b/arch/alpha/include/asm/cmpxchg.h
index ae1b96479d0c..b4b8dac759c4 100644
--- a/arch/alpha/include/asm/cmpxchg.h
+++ b/arch/alpha/include/asm/cmpxchg.h
@@ -234,7 +234,7 @@ ____cmpxchg(volatile void *ptr, unsigned long old, unsigned long new,

#define arch_cmpxchg_local(ptr, o, n) \
({ \
- __typeof__(*(ptr)) _o_ = (o); \
+ __typeof__(*(ptr)) _o_ = (__typeof__(*(ptr)))(o); \
__typeof__(*(ptr)) _n_ = (n); \
(__typeof__(*(ptr))) ____cmpxchg((ptr), (unsigned long)_o_, \
(unsigned long)_n_, \
@@ -265,7 +265,7 @@ ____cmpxchg(volatile void *ptr, unsigned long old, unsigned long new,
#define arch_cmpxchg(ptr, o, n) \
({ \
__typeof__(*(ptr)) __ret; \
- __typeof__(*(ptr)) _o_ = (o); \
+ __typeof__(*(ptr)) _o_ = (__typeof__(*(ptr)))(o); \
__typeof__(*(ptr)) _n_ = (n); \
smp_mb(); \
__ret = (__typeof__(*(ptr))) ____cmpxchg((ptr), \
diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h
index 76f43db0890f..9637e2cdb5b1 100644
--- a/arch/arc/include/asm/cmpxchg.h
+++ b/arch/arc/include/asm/cmpxchg.h
@@ -42,7 +42,7 @@
#define arch_cmpxchg_relaxed(ptr, old, new) \
({ \
__typeof__(ptr) _p_ = (ptr); \
- __typeof__(*(ptr)) _o_ = (old); \
+ __typeof__(*(ptr)) _o_ = (__typeof__(*(ptr)))(old); \
__typeof__(*(ptr)) _n_ = (new); \
__typeof__(*(ptr)) _prev_; \
\
@@ -64,7 +64,7 @@
#define arch_cmpxchg(ptr, old, new) \
({ \
volatile __typeof__(ptr) _p_ = (ptr); \
- __typeof__(*(ptr)) _o_ = (old); \
+ __typeof__(*(ptr)) _o_ = (__typeof__(*(ptr)))(old); \
__typeof__(*(ptr)) _n_ = (new); \
__typeof__(*(ptr)) _prev_; \
unsigned long __flags; \
diff --git a/arch/hexagon/include/asm/cmpxchg.h b/arch/hexagon/include/asm/cmpxchg.h
index 9c58fb81f7fd..7e117964cb6b 100644
--- a/arch/hexagon/include/asm/cmpxchg.h
+++ b/arch/hexagon/include/asm/cmpxchg.h
@@ -54,7 +54,7 @@ __arch_xchg(unsigned long x, volatile void *ptr, int size)
#define arch_cmpxchg(ptr, old, new) \
({ \
__typeof__(ptr) __ptr = (ptr); \
- __typeof__(*(ptr)) __old = (old); \
+ __typeof__(*(ptr)) __old = (__typeof__(*(ptr)))(old); \
__typeof__(*(ptr)) __new = (new); \
__typeof__(*(ptr)) __oldval = (__typeof__(*(ptr))) 0; \
\
diff --git a/arch/parisc/include/asm/cmpxchg.h b/arch/parisc/include/asm/cmpxchg.h
index bf0a0f1189eb..2a87942227f8 100644
--- a/arch/parisc/include/asm/cmpxchg.h
+++ b/arch/parisc/include/asm/cmpxchg.h
@@ -78,7 +78,7 @@ __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new_, int size)

#define arch_cmpxchg(ptr, o, n) \
({ \
- __typeof__(*(ptr)) _o_ = (o); \
+ __typeof__(*(ptr)) _o_ = (__typeof__(*(ptr)))(o); \
__typeof__(*(ptr)) _n_ = (n); \
(__typeof__(*(ptr))) __cmpxchg((ptr), (unsigned long)_o_, \
(unsigned long)_n_, sizeof(*(ptr))); \
diff --git a/arch/powerpc/include/asm/cmpxchg.h b/arch/powerpc/include/asm/cmpxchg.h
index dbb50c06f0bf..36d113b13623 100644
--- a/arch/powerpc/include/asm/cmpxchg.h
+++ b/arch/powerpc/include/asm/cmpxchg.h
@@ -698,7 +698,7 @@ __cmpxchg_acquire(void *ptr, unsigned long old, unsigned long new,
}
#define arch_cmpxchg(ptr, o, n) \
({ \
- __typeof__(*(ptr)) _o_ = (o); \
+ __typeof__(*(ptr)) _o_ = (__typeof__(*(ptr)))(o); \
__typeof__(*(ptr)) _n_ = (n); \
(__typeof__(*(ptr))) __cmpxchg((ptr), (unsigned long)_o_, \
(unsigned long)_n_, sizeof(*(ptr))); \
@@ -707,7 +707,7 @@ __cmpxchg_acquire(void *ptr, unsigned long old, unsigned long new,

#define arch_cmpxchg_local(ptr, o, n) \
({ \
- __typeof__(*(ptr)) _o_ = (o); \
+ __typeof__(*(ptr)) _o_ = (__typeof__(*(ptr)))(o); \
__typeof__(*(ptr)) _n_ = (n); \
(__typeof__(*(ptr))) __cmpxchg_local((ptr), (unsigned long)_o_, \
(unsigned long)_n_, sizeof(*(ptr))); \
@@ -715,7 +715,7 @@ __cmpxchg_acquire(void *ptr, unsigned long old, unsigned long new,

#define arch_cmpxchg_relaxed(ptr, o, n) \
({ \
- __typeof__(*(ptr)) _o_ = (o); \
+ __typeof__(*(ptr)) _o_ = (__typeof__(*(ptr)))(o); \
__typeof__(*(ptr)) _n_ = (n); \
(__typeof__(*(ptr))) __cmpxchg_relaxed((ptr), \
(unsigned long)_o_, (unsigned long)_n_, \
@@ -724,7 +724,7 @@ __cmpxchg_acquire(void *ptr, unsigned long old, unsigned long new,

#define arch_cmpxchg_acquire(ptr, o, n) \
({ \
- __typeof__(*(ptr)) _o_ = (o); \
+ __typeof__(*(ptr)) _o_ = (__typeof__(*(ptr)))(o); \
__typeof__(*(ptr)) _n_ = (n); \
(__typeof__(*(ptr))) __cmpxchg_acquire((ptr), \
(unsigned long)_o_, (unsigned long)_n_, \
diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
index 8712cf9c69dc..3026a9ff43c4 100644
--- a/arch/riscv/include/asm/cmpxchg.h
+++ b/arch/riscv/include/asm/cmpxchg.h
@@ -215,7 +215,7 @@
cas_prepend, cas_append) \
({ \
__typeof__(ptr) __ptr = (ptr); \
- __typeof__(*(__ptr)) __old = (old); \
+ __typeof__(*(__ptr)) __old = (__typeof__(*(__ptr)))(old); \
__typeof__(*(__ptr)) __new = (new); \
__typeof__(*(__ptr)) __ret; \
\
@@ -331,7 +331,7 @@ union __u128_halves {

#define __arch_cmpxchg128(p, o, n, cas_sfx) \
({ \
- __typeof__(*(p)) __o = (o); \
+ __typeof__(*(p)) __o = (__typeof__(*(p)))(o); \
union __u128_halves __hn = { .full = (n) }; \
union __u128_halves __ho = { .full = (__o) }; \
register unsigned long t1 asm ("t1") = __hn.low; \
diff --git a/arch/sh/include/asm/cmpxchg.h b/arch/sh/include/asm/cmpxchg.h
index 1e5dc5ccf7bf..7fb9214d246f 100644
--- a/arch/sh/include/asm/cmpxchg.h
+++ b/arch/sh/include/asm/cmpxchg.h
@@ -68,7 +68,7 @@ static inline unsigned long __cmpxchg(volatile void * ptr, unsigned long old,

#define arch_cmpxchg(ptr,o,n) \
({ \
- __typeof__(*(ptr)) _o_ = (o); \
+ __typeof__(*(ptr)) _o_ = (__typeof__(*(ptr)))(o); \
__typeof__(*(ptr)) _n_ = (n); \
(__typeof__(*(ptr))) __cmpxchg((ptr), (unsigned long)_o_, \
(unsigned long)_n_, sizeof(*(ptr))); \
diff --git a/arch/sparc/include/asm/cmpxchg_32.h b/arch/sparc/include/asm/cmpxchg_32.h
index 8c1a3ca34eeb..f733bd5b5d3b 100644
--- a/arch/sparc/include/asm/cmpxchg_32.h
+++ b/arch/sparc/include/asm/cmpxchg_32.h
@@ -55,7 +55,7 @@ __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new_, int size)

#define arch_cmpxchg(ptr, o, n) \
({ \
- __typeof__(*(ptr)) _o_ = (o); \
+ __typeof__(*(ptr)) _o_ = (__typeof__(*(ptr)))(o); \
__typeof__(*(ptr)) _n_ = (n); \
(__typeof__(*(ptr))) __cmpxchg((ptr), (unsigned long)_o_, \
(unsigned long)_n_, sizeof(*(ptr))); \
diff --git a/arch/sparc/include/asm/cmpxchg_64.h b/arch/sparc/include/asm/cmpxchg_64.h
index 3de25262c411..4a9ccf80487a 100644
--- a/arch/sparc/include/asm/cmpxchg_64.h
+++ b/arch/sparc/include/asm/cmpxchg_64.h
@@ -170,7 +170,7 @@ __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new, int size)

#define arch_cmpxchg(ptr,o,n) \
({ \
- __typeof__(*(ptr)) _o_ = (o); \
+ __typeof__(*(ptr)) _o_ = (__typeof__(*(ptr)))(o); \
__typeof__(*(ptr)) _n_ = (n); \
(__typeof__(*(ptr))) __cmpxchg((ptr), (unsigned long)_o_, \
(unsigned long)_n_, sizeof(*(ptr))); \
diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h
index a88b06f1c35e..a7984842c58a 100644
--- a/arch/x86/include/asm/cmpxchg.h
+++ b/arch/x86/include/asm/cmpxchg.h
@@ -85,7 +85,7 @@ extern void __add_wrong_size(void)
#define __raw_cmpxchg(ptr, old, new, size, lock) \
({ \
__typeof__(*(ptr)) __ret; \
- __typeof__(*(ptr)) __old = (old); \
+ __typeof__(*(ptr)) __old = (__typeof__(*(ptr)))(old); \
__typeof__(*(ptr)) __new = (new); \
switch (size) { \
case __X86_CASE_B: \
diff --git a/arch/xtensa/include/asm/cmpxchg.h b/arch/xtensa/include/asm/cmpxchg.h
index b6db4838b175..0f68eddb6d2d 100644
--- a/arch/xtensa/include/asm/cmpxchg.h
+++ b/arch/xtensa/include/asm/cmpxchg.h
@@ -83,7 +83,7 @@ __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new, int size)
}

#define arch_cmpxchg(ptr,o,n) \
- ({ __typeof__(*(ptr)) _o_ = (o); \
+ ({ __typeof__(*(ptr)) _o_ = (__typeof__(*(ptr)))(o); \
__typeof__(*(ptr)) _n_ = (n); \
(__typeof__(*(ptr))) __cmpxchg((ptr), (unsigned long)_o_, \
(unsigned long)_n_, sizeof (*(ptr))); \
diff --git a/tools/arch/x86/include/asm/cmpxchg.h b/tools/arch/x86/include/asm/cmpxchg.h
index 0ed9ca2766ad..b71938db9422 100644
--- a/tools/arch/x86/include/asm/cmpxchg.h
+++ b/tools/arch/x86/include/asm/cmpxchg.h
@@ -35,7 +35,7 @@ extern void __cmpxchg_wrong_size(void)
#define __raw_cmpxchg(ptr, old, new, size, lock) \
({ \
__typeof__(*(ptr)) __ret; \
- __typeof__(*(ptr)) __old = (old); \
+ __typeof__(*(ptr)) __old = (__typeof__(*(ptr)))(old); \
__typeof__(*(ptr)) __new = (new); \
switch (size) { \
case __X86_CASE_B: \

---
base-commit: 48b3cd69265f346f64b93064723492da46206e9b
change-id: 20260402-cmpxchg-2fd307f44a41

Best regards,
--
Hangbin Liu <liuhangbin@gmail.com>