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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-483051626c1sm8323325e9.13.2026.02.02.11.28.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 02 Feb 2026 11:28:19 -0800 (PST) Date: Mon, 2 Feb 2026 19:28:18 +0000 From: David Laight To: Will Deacon Cc: Marco Elver , Peter Zijlstra , 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 Subject: Re: [PATCH v3 2/3] arm64: Optimize __READ_ONCE() with CONFIG_LTO=y Message-ID: <20260202192818.3002fe7a@pumpkin> In-Reply-To: References: <20260130132951.2714396-1-elver@google.com> <20260130132951.2714396-3-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 Mon, 2 Feb 2026 15:36:40 +0000 Will Deacon wrote: > On Fri, Jan 30, 2026 at 02:28:25PM +0100, Marco Elver wrote: > > Rework arm64 LTO __READ_ONCE() to improve code generation as follows: ... \ > > default: \ > > - atomic = 0; \ > > + __u.__val = *(volatile typeof(*__x) *)__x; \ > > Since we're not providing acquire semantics for the non-atomic case, > what we really want is the generic definition of __READ_ONCE() from > include/asm-generic/rwonce.h here. The header inclusion mess prevents > that, but why can't we just inline that definition here for the > 'default' case? If TYPEOF_UNQUAL() leads to better codegen, shouldn't > we use that to implement __unqual_scalar_typeof() when it is available? > > I fear I'm missing something here, but it just feels like we're > optimising a pretty niche case (arm64 + LTO + non-atomic __READ_ONCE()) > in a way that looks more generally applicable. Is that path even needed? I'm sure I've built an x86-64 allmodconfig with it being an error. If you look back in the history it used to be an error. Anything to simplify READ_ONCE() will noticeably speed up build times. Even on x86 just removing the check that the size is 1, 2, 4 or 8 makes a measurable difference - and that check doesn't need to be done on every compile. I'm not setup to do arm builds - never mind LTO ones. (Yes, I know, it 'just' involves downloading the toolchain.) David