[PATCH v2] cmpxchg: allow const-qualified old value in cmpxchg()
From: Hangbin Liu
Date: Thu Apr 02 2026 - 21:26:53 EST
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.
Fix this by using __auto_type with the conditional expression trick:
__auto_type _o_ = 1 ? (o) : *(ptr);
The __auto_type deduces the type from the expression, so _o_ naturally
takes the const-qualified type when o is const-qualified, avoiding the
discarded-qualifiers warning. The conditional expression forces the
compiler to verify that o and *(ptr) have compatible types, preserving
the type-safety check that a bare __auto_type _o_ = (o) would lose.
Since the condition is the constant 1, the compiler optimizes away the
*(ptr) branch entirely with no runtime cost.
For macros where ptr is already captured in a local variable (arc's
_p_, hexagon's __ptr, riscv's __ptr), the captured variable is used
in the conditional expression to avoid re-evaluating ptr.
The new value is intentionally left without this treatment: 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@xxxxxxxxxx>
Signed-off-by: Hangbin Liu <liuhangbin@xxxxxxxxx>
---
Changes in v2:
- Use __auto_type as the previous way will loose ptr type checking (David Laight)
- Link to v1: https://lore.kernel.org/r/20260402-cmpxchg-v1-1-4561e2c05d2c@xxxxxxxxx
---
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..e6cb60ca35bf 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); \
+ __auto_type _o_ = 1 ? (o) : *(ptr); \
__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); \
+ __auto_type _o_ = 1 ? (o) : *(ptr); \
__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..e18609174133 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); \
+ __auto_type _o_ = 1 ? (old) : *(_p_); \
__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); \
+ __auto_type _o_ = 1 ? (old) : *(_p_); \
__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..88986fe04072 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); \
+ __auto_type __old = 1 ? (old) : *(__ptr); \
__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..30748e31aa17 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); \
+ __auto_type _o_ = 1 ? (o) : *(ptr); \
__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..eaf88b7d0443 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); \
+ __auto_type _o_ = 1 ? (o) : *(ptr); \
__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); \
+ __auto_type _o_ = 1 ? (o) : *(ptr); \
__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); \
+ __auto_type _o_ = 1 ? (o) : *(ptr); \
__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); \
+ __auto_type _o_ = 1 ? (o) : *(ptr); \
__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..30ade671298f 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); \
+ __auto_type __old = 1 ? (old) : *(__ptr); \
__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); \
+ __auto_type __o = 1 ? (o) : *(p); \
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..6c1bfb367f11 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); \
+ __auto_type _o_ = 1 ? (o) : *(ptr); \
__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..df40a16dd021 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); \
+ __auto_type _o_ = 1 ? (o) : *(ptr); \
__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..4b540405fcc7 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); \
+ __auto_type _o_ = 1 ? (o) : *(ptr); \
__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..e0340579ef69 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); \
+ __auto_type __old = 1 ? (old) : *(ptr); \
__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..057fd24ed125 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); \
+ ({ __auto_type _o_ = 1 ? (o) : *(ptr); \
__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..bbd3960c2218 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); \
+ __auto_type __old = 1 ? (old) : *(ptr); \
__typeof__(*(ptr)) __new = (new); \
switch (size) { \
case __X86_CASE_B: \
---
base-commit: f8f5627a8aeab15183eef8930bf75ba88a51622f
change-id: 20260402-cmpxchg-2fd307f44a41
Best regards,
--
Hangbin Liu <liuhangbin@xxxxxxxxx>