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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4887e735532sm217845265e9.0.2026.04.02.01.57.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 02 Apr 2026 01:57:49 -0700 (PDT) Date: Thu, 2 Apr 2026 09:57:47 +0100 From: David Laight To: Hangbin Liu Cc: 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 , 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 Subject: Re: [PATCH] cmpxchg: allow const-qualified old value in cmpxchg() Message-ID: <20260402095747.6efbd456@pumpkin> In-Reply-To: <20260402-cmpxchg-v1-1-4561e2c05d2c@gmail.com> References: <20260402-cmpxchg-v1-1-4561e2c05d2c@gmail.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: sparclinux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Thu, 02 Apr 2026 14:56:01 +0800 Hangbin Liu wrote: > 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 > Signed-off-by: Hangbin Liu ... > > 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_, \ That looks like it loses the check that 'o' has the same type as '*ptr'. Maybe this works? auto _o_ = 1 ? (o) : *(ptr); David