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Tue, 06 Jan 2026 04:51:53 -0800 (PST) Date: Tue, 6 Jan 2026 12:51:52 +0000 In-Reply-To: <20260106124326.GY3707891@noisy.programming.kicks-ass.net> Mime-Version: 1.0 References: <20251231-rwonce-v1-0-702a10b85278@google.com> <20251231-rwonce-v1-3-702a10b85278@google.com> <20260106124326.GY3707891@noisy.programming.kicks-ass.net> Message-ID: Subject: Re: [PATCH 3/5] rust: sync: support using bool with READ_ONCE From: Alice Ryhl To: Peter Zijlstra Cc: Boqun Feng , Will Deacon , Richard Henderson , Matt Turner , Magnus Lindholm , Catalin Marinas , Miguel Ojeda , Gary Guo , "=?utf-8?B?QmrDtnJu?= Roy Baron" , Benno Lossin , Andreas Hindborg , Trevor Gross , Danilo Krummrich , Mark Rutland , FUJITA Tomonori , Frederic Weisbecker , Lyude Paul , Thomas Gleixner , Anna-Maria Behnsen , John Stultz , Stephen Boyd , Alexander Viro , Christian Brauner , Jan Kara , linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, rust-for-linux@vger.kernel.org, linux-fsdevel@vger.kernel.org Content-Type: text/plain; charset="utf-8" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260106_045155_408062_F88D4B45 X-CRM114-Status: GOOD ( 17.45 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Jan 06, 2026 at 01:43:26PM +0100, Peter Zijlstra wrote: > On Wed, Dec 31, 2025 at 12:22:27PM +0000, Alice Ryhl wrote: > > +/// Read an integer as a boolean once. > > +/// > > +/// Returns `true` if the value behind the pointer is non-zero. Otherwise returns `false`. > > +/// > > +/// # Safety > > +/// > > +/// It must be safe to `READ_ONCE` the `ptr` with type `u8`. > > +#[inline(always)] > > +#[track_caller] > > +unsafe fn read_once_bool(ptr: *const bool) -> bool { > > + // Implement `read_once_bool` in terms of `read_once_1`. The arch-specific logic is inside > > + // of `read_once_1`. > > + // > > + // SAFETY: It is safe to `READ_ONCE` the `ptr` with type `u8`. > > + let byte = unsafe { read_once_1(ptr.cast::()) }; > > + byte != 0u8 > > +} > > Does this hardcode that sizeof(_Bool) == 1? There are ABIs where this is > not the case. Hm, it hardcodes that the Rust type called bool is sizeof(_) == 1. Presumably bindgen will not translate _Bool to bool when it appears in C types on such platforms. But I don't really know - I have not looked into this case. Alice