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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-435e10e4762sm22183847f8f.6.2026.01.30.07.06.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 30 Jan 2026 07:06:59 -0800 (PST) Date: Fri, 30 Jan 2026 15:06:58 +0000 From: David Laight To: Marco Elver Cc: Peter Zijlstra , Will Deacon , Ingo Molnar , Thomas Gleixner , Boqun Feng , Waiman Long , Bart Van Assche , llvm@lists.linux.dev, Catalin Marinas , Arnd Bergmann , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Boqun Feng Subject: Re: [PATCH v3 1/3] arm64: Fix non-atomic __READ_ONCE() with CONFIG_LTO=y Message-ID: <20260130150658.617b65ad@pumpkin> In-Reply-To: <20260130132951.2714396-2-elver@google.com> References: <20260130132951.2714396-1-elver@google.com> <20260130132951.2714396-2-elver@google.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@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 Fri, 30 Jan 2026 14:28:24 +0100 Marco Elver wrote: > The implementation of __READ_ONCE() under CONFIG_LTO=y incorrectly > qualified the fallback "once" access for types larger than 8 bytes, > which are not atomic but should still happen "once" and suppress common > compiler optimizations. > > The cast `volatile typeof(__x)` applied the volatile qualifier to the > pointer type itself rather than the pointee. This created a volatile > pointer to a non-volatile type, which violated __READ_ONCE() semantics. > > Fix this by casting to `volatile typeof(*__x) *`. > > With a defconfig + LTO + debug options build, we see the following > functions to be affected: > > xen_manage_runstate_time (884 -> 944 bytes) > xen_steal_clock (248 -> 340 bytes) > ^-- use __READ_ONCE() to load vcpu_runstate_info structs > > Fixes: e35123d83ee3 ("arm64: lto: Strengthen READ_ONCE() to acquire when CONFIG_LTO=y") > Cc: > Reviewed-by: Boqun Feng > Signed-off-by: Marco Elver I found this in some testing (on godbolt), so: Tested-by: David Laight > --- > arch/arm64/include/asm/rwonce.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h > index 78beceec10cd..fc0fb42b0b64 100644 > --- a/arch/arm64/include/asm/rwonce.h > +++ b/arch/arm64/include/asm/rwonce.h > @@ -58,7 +58,7 @@ > default: \ > atomic = 0; \ > } \ > - atomic ? (typeof(*__x))__u.__val : (*(volatile typeof(__x))__x);\ > + atomic ? (typeof(*__x))__u.__val : (*(volatile typeof(*__x) *)__x);\ > }) > > #endif /* !BUILD_VDSO */