[{"id":3672621,"web_url":"http://patchwork.ozlabs.org/comment/3672621/","msgid":"<20260402095747.6efbd456@pumpkin>","date":"2026-04-02T08:57:47","subject":"Re: [PATCH] cmpxchg: allow const-qualified old value in cmpxchg()","submitter":{"id":89989,"url":"http://patchwork.ozlabs.org/api/people/89989/","name":"David Laight","email":"david.laight.linux@gmail.com"},"content":"On Thu, 02 Apr 2026 14:56:01 +0800\nHangbin Liu wrote:\n\n> The old value passed to cmpxchg() is semantically read-only: it is\n> only loaded into a register as a comparand and is never written back.\n> However, the macro currently assigns it implicitly to a local variable\n> of type __typeof__(*(ptr)), which triggers -Werror=discarded-qualifiers\n> when old is a const-qualified pointer and ptr points to a non-const type.\n> \n> To avoid this, let's add an explicit cast to __typeof__(*(ptr)) for the\n> old local variable in the cmpxchg macros. This explicit cast suppresses\n> the -Wdiscarded-qualifiers diagnostic.\n> \n> The new value is intentionally left without a cast: new will be stored\n> into *ptr, so silently accepting a const-qualified new would allow\n> callers to store a pointer-to-const into a non-const location without\n> any compiler warning.\n> \n> Suggested-by: Jakub Kicinski \n> Signed-off-by: Hangbin Liu \n...\n> \n> diff --git a/arch/alpha/include/asm/cmpxchg.h b/arch/alpha/include/asm/cmpxchg.h\n> index ae1b96479d0c..b4b8dac759c4 100644\n> --- a/arch/alpha/include/asm/cmpxchg.h\n> +++ b/arch/alpha/include/asm/cmpxchg.h\n> @@ -234,7 +234,7 @@ ____cmpxchg(volatile void *ptr, unsigned long old, unsigned long new,\n> \n> #define arch_cmpxchg_local(ptr, o, n)\t\t\t\t\t\\\n> ({\t\t\t\t\t\t\t\t\t\\\n> -\t__typeof__(*(ptr)) _o_ = (o);\t\t\t\t\t\\\n> +\t__typeof__(*(ptr)) _o_ = (__typeof__(*(ptr)))(o);\t\t\\\n> \t__typeof__(*(ptr)) _n_ = (n);\t\t\t\t\t\\\n> \t(__typeof__(*(ptr))) ____cmpxchg((ptr), (unsigned long)_o_,\t\\\n> \t\t\t\t\t (unsigned long)_n_,\t\t\\\n\nThat looks like it loses the check that 'o' has the same type as '*ptr'.\nMaybe this works?\n\tauto _o_ = 1 ? (o) : *(ptr);\n\n David","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linuxppc-dev@lists.ozlabs.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=s7OZk4Ay;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=lists.ozlabs.org\n (client-ip=2404:9400:21b9:f100::1; helo=lists.ozlabs.org;\n envelope-from=linuxppc-dev+bounces-19209-incoming=patchwork.ozlabs.org@lists.ozlabs.org;\n receiver=patchwork.ozlabs.org)","lists.ozlabs.org;\n arc=none smtp.remote-ip=\"2a00:1450:4864:20::336\"","lists.ozlabs.org;\n dmarc=pass (p=none dis=none) header.from=gmail.com","lists.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=s7OZk4Ay;\n\tdkim-atps=neutral","lists.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gmail.com\n (client-ip=2a00:1450:4864:20::336; helo=mail-wm1-x336.google.com;\n envelope-from=david.laight.linux@gmail.com; receiver=lists.ozlabs.org)"],"Received":["from lists.ozlabs.org (lists.ozlabs.org\n [IPv6:2404:9400:21b9:f100::1])\n\t(using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)\n\t key-exchange x25519 server-signature ECDSA (secp384r1 raw public key)\n server-digest SHA384)\n\t(No client certificate requested)\n\tby legolas.ozlabs.org (Postfix) with ESMTPS id 4fmbNW6nhDz1yCs\n\tfor ; Thu, 02 Apr 2026 19:58:03 +1100 (AEDT)","from boromir.ozlabs.org (localhost [127.0.0.1])\n\tby lists.ozlabs.org (Postfix) with ESMTP id 4fmbNV3VmQz2yYK;\n\tThu, 02 Apr 2026 19:58:02 +1100 (AEDT)","from mail-wm1-x336.google.com (mail-wm1-x336.google.com\n [IPv6:2a00:1450:4864:20::336])\n\t(using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)\n\t key-exchange x25519 server-signature RSA-PSS (2048 bits) server-digest\n SHA256)\n\t(No client certificate requested)\n\tby lists.ozlabs.org (Postfix) with ESMTPS id 4fmbNR6rydz2xls\n\tfor ; Thu, 02 Apr 2026 19:57:58 +1100 (AEDT)","by mail-wm1-x336.google.com with SMTP id\n 5b1f17b1804b1-48334ee0aeaso5220115e9.1\n for ;\n Thu, 02 Apr 2026 01:57:58 -0700 (PDT)","from pumpkin (82-69-66-36.dsl.in-addr.zen.co.uk. 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Bottomley\"\n , Helge Deller ,\n Madhavan Srinivasan , Michael Ellerman\n , Nicholas Piggin , \"Christophe\n Leroy (CS GROUP)\" , Paul Walmsley ,\n Palmer Dabbelt , Albert Ou ,\n Alexandre Ghiti , Yoshinori Sato\n , Rich Felker , John Paul\n Adrian Glaubitz , \"David S. Miller\"\n , Andreas Larsson , Thomas\n Gleixner , Ingo Molnar , Borislav Petkov\n , Dave Hansen , x86@kernel.org,\n \"H. Peter Anvin\" , Chris Zankel , Max\n Filippov , linux-alpha@vger.kernel.org,\n linux-kernel@vger.kernel.org, linux-snps-arc@lists.infradead.org,\n linux-hexagon@vger.kernel.org, linux-parisc@vger.kernel.org,\n linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,\n linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, Jakub Kicinski\n ","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)","X-Mailing-List":"linuxppc-dev@lists.ozlabs.org","List-Id":"","List-Help":"","List-Owner":"","List-Post":"","List-Archive":",\n ","List-Subscribe":",\n ,\n ","List-Unsubscribe":"","Precedence":"list","MIME-Version":"1.0","Content-Type":"text/plain; charset=US-ASCII","Content-Transfer-Encoding":"7bit","X-Spam-Status":"No, score=-0.2 required=3.0 tests=DKIM_SIGNED,DKIM_VALID,\n\tDKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM,RCVD_IN_DNSWL_NONE,\n\tSPF_HELO_NONE,SPF_PASS autolearn=disabled version=4.0.1 OzLabs 8","X-Spam-Checker-Version":"SpamAssassin 4.0.1 (2024-03-25) on lists.ozlabs.org"}},{"id":3672659,"web_url":"http://patchwork.ozlabs.org/comment/3672659/","msgid":"<20260402104733.GZ3738786@noisy.programming.kicks-ass.net>","date":"2026-04-02T10:47:33","subject":"Re: [PATCH] cmpxchg: allow const-qualified old value in cmpxchg()","submitter":{"id":493,"url":"http://patchwork.ozlabs.org/api/people/493/","name":"Peter Zijlstra","email":"peterz@infradead.org"},"content":"On Thu, Apr 02, 2026 at 02:56:01PM +0800, Hangbin Liu wrote:\n> The old value passed to cmpxchg() is semantically read-only: it is\n> only loaded into a register as a comparand and is never written back.\n> However, the macro currently assigns it implicitly to a local variable\n> of type __typeof__(*(ptr)), which triggers -Werror=discarded-qualifiers\n> when old is a const-qualified pointer and ptr points to a non-const type.\n> \n> To avoid this, let's add an explicit cast to __typeof__(*(ptr)) for the\n> old local variable in the cmpxchg macros. This explicit cast suppresses\n> the -Wdiscarded-qualifiers diagnostic.\n> \n> The new value is intentionally left without a cast: new will be stored\n> into *ptr, so silently accepting a const-qualified new would allow\n> callers to store a pointer-to-const into a non-const location without\n> any compiler warning.\n> \n> Suggested-by: Jakub Kicinski \n> Signed-off-by: Hangbin Liu \n> ---\n\nWhere is this a problem? I've never seen this. Also, new code should\nprobably use try_cmpxchg() anyway.","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linuxppc-dev@lists.ozlabs.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n secure) header.d=infradead.org header.i=@infradead.org header.a=rsa-sha256\n header.s=desiato.20200630 header.b=fWHvfBdk;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=lists.ozlabs.org\n (client-ip=2404:9400:21b9:f100::1; helo=lists.ozlabs.org;\n envelope-from=linuxppc-dev+bounces-19211-incoming=patchwork.ozlabs.org@lists.ozlabs.org;\n receiver=patchwork.ozlabs.org)","lists.ozlabs.org;\n arc=none smtp.remote-ip=\"2001:8b0:10b:1:d65d:64ff:fe57:4e05\"","lists.ozlabs.org;\n dmarc=pass (p=none dis=none) header.from=infradead.org","lists.ozlabs.org;\n\tdkim=pass (2048-bit key;\n secure) header.d=infradead.org header.i=@infradead.org header.a=rsa-sha256\n header.s=desiato.20200630 header.b=fWHvfBdk;\n\tdkim-atps=neutral","lists.ozlabs.org;\n spf=none (no SPF record) smtp.mailfrom=infradead.org\n (client-ip=2001:8b0:10b:1:d65d:64ff:fe57:4e05; 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Bottomley\" ,\n\tHelge Deller ,\n\tMadhavan Srinivasan ,\n\tMichael Ellerman ,\n\tNicholas Piggin ,\n\t\"Christophe Leroy (CS GROUP)\" ,\n\tPaul Walmsley , Palmer Dabbelt ,\n\tAlbert Ou , Alexandre Ghiti ,\n\tYoshinori Sato ,\n\tRich Felker ,\n\tJohn Paul Adrian Glaubitz ,\n\t\"David S. Miller\" ,\n\tAndreas Larsson ,\n\tThomas Gleixner , Ingo Molnar ,\n\tBorislav Petkov ,\n\tDave Hansen , x86@kernel.org,\n\t\"H. Peter Anvin\" , Chris Zankel ,\n\tMax Filippov , linux-alpha@vger.kernel.org,\n\tlinux-kernel@vger.kernel.org, linux-snps-arc@lists.infradead.org,\n\tlinux-hexagon@vger.kernel.org, linux-parisc@vger.kernel.org,\n\tlinuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,\n\tlinux-sh@vger.kernel.org, sparclinux@vger.kernel.org,\n\tJakub Kicinski ","Subject":"Re: [PATCH] cmpxchg: allow const-qualified old value in cmpxchg()","Message-ID":"<20260402104733.GZ3738786@noisy.programming.kicks-ass.net>","References":"<20260402-cmpxchg-v1-1-4561e2c05d2c@gmail.com>","X-Mailing-List":"linuxppc-dev@lists.ozlabs.org","List-Id":"","List-Help":"","List-Owner":"","List-Post":"","List-Archive":",\n ","List-Subscribe":",\n ,\n ","List-Unsubscribe":"","Precedence":"list","MIME-Version":"1.0","Content-Type":"text/plain; charset=us-ascii","Content-Disposition":"inline","In-Reply-To":"<20260402-cmpxchg-v1-1-4561e2c05d2c@gmail.com>","X-Spam-Status":"No, score=-2.5 required=3.0 tests=DKIM_SIGNED,DKIM_VALID,\n\tDKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_MED,SPF_HELO_NONE,SPF_NONE\n\tautolearn=disabled version=4.0.1 OzLabs 8","X-Spam-Checker-Version":"SpamAssassin 4.0.1 (2024-03-25) on lists.ozlabs.org"}},{"id":3672713,"web_url":"http://patchwork.ozlabs.org/comment/3672713/","msgid":"","date":"2026-04-02T13:07:45","subject":"Re: [PATCH] cmpxchg: allow const-qualified old value in cmpxchg()","submitter":{"id":15639,"url":"http://patchwork.ozlabs.org/api/people/15639/","name":"Hangbin Liu","email":"liuhangbin@gmail.com"},"content":"On Thu, Apr 02, 2026 at 09:57:47AM +0100, David Laight wrote:\n> > diff --git a/arch/alpha/include/asm/cmpxchg.h b/arch/alpha/include/asm/cmpxchg.h\n> > index ae1b96479d0c..b4b8dac759c4 100644\n> > --- a/arch/alpha/include/asm/cmpxchg.h\n> > +++ b/arch/alpha/include/asm/cmpxchg.h\n> > @@ -234,7 +234,7 @@ ____cmpxchg(volatile void *ptr, unsigned long old, unsigned long new,\n> > \n> > #define arch_cmpxchg_local(ptr, o, n)\t\t\t\t\t\\\n> > ({\t\t\t\t\t\t\t\t\t\\\n> > -\t__typeof__(*(ptr)) _o_ = (o);\t\t\t\t\t\\\n> > +\t__typeof__(*(ptr)) _o_ = (__typeof__(*(ptr)))(o);\t\t\\\n> > \t__typeof__(*(ptr)) _n_ = (n);\t\t\t\t\t\\\n> > \t(__typeof__(*(ptr))) ____cmpxchg((ptr), (unsigned long)_o_,\t\\\n> > \t\t\t\t\t (unsigned long)_n_,\t\t\\\n> \n> That looks like it loses the check that 'o' has the same type as '*ptr'.\n> Maybe this works?\n> \tauto _o_ = 1 ? (o) : *(ptr);\n> \nOh, I didn't notice this. Thanks for the comment!\n\nBest Regards\nHangbin","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linuxppc-dev@lists.ozlabs.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=aYMrR4Co;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=lists.ozlabs.org\n (client-ip=2404:9400:21b9:f100::1; helo=lists.ozlabs.org;\n envelope-from=linuxppc-dev+bounces-19216-incoming=patchwork.ozlabs.org@lists.ozlabs.org;\n receiver=patchwork.ozlabs.org)","lists.ozlabs.org;\n arc=none smtp.remote-ip=\"2607:f8b0:4864:20::1034\"","lists.ozlabs.org;\n dmarc=pass (p=none dis=none) header.from=gmail.com","lists.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=aYMrR4Co;\n\tdkim-atps=neutral","lists.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gmail.com\n (client-ip=2607:f8b0:4864:20::1034; 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Bottomley\" ,\n\tHelge Deller ,\n\tMadhavan Srinivasan ,\n\tMichael Ellerman ,\n\tNicholas Piggin ,\n\t\"Christophe Leroy (CS GROUP)\" ,\n\tPaul Walmsley , Palmer Dabbelt ,\n\tAlbert Ou , Alexandre Ghiti ,\n\tYoshinori Sato ,\n\tRich Felker ,\n\tJohn Paul Adrian Glaubitz ,\n\t\"David S. Miller\" ,\n\tAndreas Larsson ,\n\tThomas Gleixner , Ingo Molnar ,\n\tBorislav Petkov ,\n\tDave Hansen , x86@kernel.org,\n\t\"H. Peter Anvin\" , Chris Zankel ,\n\tMax Filippov , linux-alpha@vger.kernel.org,\n\tlinux-kernel@vger.kernel.org, linux-snps-arc@lists.infradead.org,\n\tlinux-hexagon@vger.kernel.org, linux-parisc@vger.kernel.org,\n\tlinuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,\n\tlinux-sh@vger.kernel.org, sparclinux@vger.kernel.org,\n\tJakub Kicinski ","Subject":"Re: [PATCH] cmpxchg: allow const-qualified old value in cmpxchg()","Message-ID":"","References":"<20260402-cmpxchg-v1-1-4561e2c05d2c@gmail.com>\n <20260402095747.6efbd456@pumpkin>","X-Mailing-List":"linuxppc-dev@lists.ozlabs.org","List-Id":"","List-Help":"","List-Owner":"","List-Post":"","List-Archive":",\n ","List-Subscribe":",\n ,\n ","List-Unsubscribe":"","Precedence":"list","MIME-Version":"1.0","Content-Type":"text/plain; charset=us-ascii","Content-Disposition":"inline","In-Reply-To":"<20260402095747.6efbd456@pumpkin>","X-Spam-Status":"No, score=-0.2 required=3.0 tests=DKIM_SIGNED,DKIM_VALID,\n\tDKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM,RCVD_IN_DNSWL_NONE,\n\tSPF_HELO_NONE,SPF_PASS autolearn=disabled version=4.0.1 OzLabs 8","X-Spam-Checker-Version":"SpamAssassin 4.0.1 (2024-03-25) on lists.ozlabs.org"}}]