linux-kernel - Re: [PATCH 3/5] rust: sync: support using bool with READ

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Message-ID: <aV0FaEjA7fe3NPAX@google.com>
Date: Tue, 6 Jan 2026 12:51:52 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Boqun Feng <boqun.feng@...il.com>, Will Deacon <will@...nel.org>, 
	Richard Henderson <richard.henderson@...aro.org>, Matt Turner <mattst88@...il.com>, 
	Magnus Lindholm <linmag7@...il.com>, Catalin Marinas <catalin.marinas@....com>, 
	Miguel Ojeda <ojeda@...nel.org>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	FUJITA Tomonori <fujita.tomonori@...il.com>, Frederic Weisbecker <frederic@...nel.org>, 
	Lyude Paul <lyude@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Anna-Maria Behnsen <anna-maria@...utronix.de>, John Stultz <jstultz@...gle.com>, 
	Stephen Boyd <sboyd@...nel.org>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org, 
	linux-alpha@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 3/5] rust: sync: support using bool with READ_ONCE

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::<u8>()) };
> > +    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