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Thu, 02 Apr 2026 18:25:59 -0700 (PDT) From: Hangbin Liu Date: Fri, 03 Apr 2026 09:19:54 +0800 Subject: [PATCH v2] cmpxchg: allow const-qualified old value in cmpxchg() X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260403-cmpxchg-v2-1-23acd2727447@gmail.com> X-B4-Tracking: v=1; b=H4sIAAAAAAAC/2XMSwrDIBSF4a2EO65Fb00KHXUfJQPxeaHGoEFSg nuvzbTD/3D4Dig2ky3wGA7ItlKhtPTAywA6qMVbRqY3IMeJS45Mx3XXwTN05sbvTkolBfT3mq2 j/ZRec+9AZUv5c8JV/NZ/owommBwnYVHz0aB++qjofdUpwtxa+wIngfApngAAAA== X-Change-ID: 20260402-cmpxchg-2fd307f44a41 To: Richard Henderson , Matt Turner , Magnus Lindholm , Vineet Gupta , Brian Cain , "James E.J. Bottomley" , Helge Deller , Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , "Christophe Leroy (CS GROUP)" , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Yoshinori Sato , Rich Felker , John Paul Adrian Glaubitz , "David S. Miller" , Andreas Larsson , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Chris Zankel , Max Filippov , David Laight Cc: linux-alpha@vger.kernel.org, linux-kernel@vger.kernel.org, linux-snps-arc@lists.infradead.org, linux-hexagon@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, Jakub Kicinski , Hangbin Liu X-Mailer: b4 0.14.3 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 Signed-off-by: Hangbin Liu --- 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@gmail.com --- 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