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[103.168.172.200]) by smtp.gmail.com with ESMTPSA id af79cd13be357-8c6b22bb7cbsm421467685a.39.2026.01.18.17.22.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 18 Jan 2026 17:22:13 -0800 (PST) Received: from phl-compute-02.internal (phl-compute-02.internal [10.202.2.42]) by mailfauth.phl.internal (Postfix) with ESMTP id 2D5A7F40079; Sun, 18 Jan 2026 20:22:13 -0500 (EST) Received: from phl-frontend-03 ([10.202.2.162]) by phl-compute-02.internal (MEProxy); Sun, 18 Jan 2026 20:22:13 -0500 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeefgedrtddtgddufeeivdefucetufdoteggodetrf dotffvucfrrhhofhhilhgvmecuhfgrshhtofgrihhlpdfurfetoffkrfgpnffqhgenuceu rghilhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddmnecujf gurhepfffhvfevuffkfhggtggujgesthdtredttddtvdenucfhrhhomhepuehoqhhunhcu hfgvnhhguceosghoqhhunhdrfhgvnhhgsehgmhgrihhlrdgtohhmqeenucggtffrrghtth gvrhhnpefhtdekvdefgedvkeefffeiiedthfelgefhkedvhefgheefveehffejgeehfeff feenucffohhmrghinhepmhhsghhiugdrlhhinhhknecuvehluhhsthgvrhfuihiivgeptd enucfrrghrrghmpehmrghilhhfrhhomhepsghoqhhunhdomhgvshhmthhprghuthhhphgv rhhsohhnrghlihhthidqieelvdeghedtieegqddujeejkeehheehvddqsghoqhhunhdrfh gvnhhgpeepghhmrghilhdrtghomhesfhhigihmvgdrnhgrmhgvpdhnsggprhgtphhtthho pedugedpmhhouggvpehsmhhtphhouhhtpdhrtghpthhtohepfhhujhhithgrrdhtohhmoh hnohhrihesghhmrghilhdrtghomhdprhgtphhtthhopehojhgvuggrsehkvghrnhgvlhdr ohhrghdprhgtphhtthhopehpvghtvghriiesihhnfhhrrgguvggrugdrohhrghdprhgtph htthhopeifihhllheskhgvrhhnvghlrdhorhhgpdhrtghpthhtoheprgdrhhhinhgusgho rhhgsehkvghrnhgvlhdrohhrghdprhgtphhtthhopegrlhhitggvrhihhhhlsehgohhogh hlvgdrtghomhdprhgtphhtthhopegsjhhorhhnfegpghhhsehprhhothhonhhmrghilhdr tghomhdprhgtphhtthhopegurghkrheskhgvrhhnvghlrdhorhhgpdhrtghpthhtohepgh grrhihsehgrghrhihguhhordhnvght X-ME-Proxy: Feedback-ID: iad51458e:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Sun, 18 Jan 2026 20:22:12 -0500 (EST) Date: Mon, 19 Jan 2026 09:22:10 +0800 From: Boqun Feng To: FUJITA Tomonori Cc: ojeda@kernel.org, peterz@infradead.org, will@kernel.org, a.hindborg@kernel.org, aliceryhl@google.com, bjorn3_gh@protonmail.com, dakr@kernel.org, gary@garyguo.net, lossin@kernel.org, mark.rutland@arm.com, tmgross@umich.edu, rust-for-linux@vger.kernel.org Subject: Re: [PATCH v4 1/2] rust: sync: atomic: Add performance-optimal-integer-backed Flag for atomic booleans Message-ID: References: <20260115021230.3297420-1-fujita.tomonori@gmail.com> <20260115021230.3297420-2-fujita.tomonori@gmail.com> Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260115021230.3297420-2-fujita.tomonori@gmail.com> On Thu, Jan 15, 2026 at 11:12:29AM +0900, FUJITA Tomonori wrote: > Add a new Flag enum (Clear/Set) and implement AtomicType for it, so > users can use Atomic for boolean flags. > > The backing integer type is an implementation detail; it may vary by > architecture and change in the future. > > Document when Atomic is generally preferable to Atomic: in > particular, when RMW operations such as xchg()/cmpxchg() may be used > and minimizing memory usage is not the top priority. On some > architectures without byte-sized RMW instructions, Atomic can be > slower for RMW operations. > > Signed-off-by: FUJITA Tomonori > --- > rust/kernel/sync/atomic.rs | 71 ++++++++++++++++++++++++++++++++++++++ > 1 file changed, 71 insertions(+) > > diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs > index ca9cab77abf0..58f57903460f 100644 > --- a/rust/kernel/sync/atomic.rs > +++ b/rust/kernel/sync/atomic.rs I prefer we can move `Flag` into atomic/predefine.rs, I queued in rust-sync with the following version, please let me know whether it works, thank you all! ------------------>8 Subject: [PATCH] rust: sync: atomic: Add performance-optimal-integer-backed Flag for atomic booleans Add a new Flag enum (Clear/Set) and implement AtomicType for it, so users can use Atomic for boolean flags. The backing integer type is an implementation detail; it may vary by architecture and change in the future. Document when Atomic is generally preferable to Atomic: in particular, when RMW operations such as xchg()/cmpxchg() may be used and minimizing memory usage is not the top priority. On some architectures without byte-sized RMW instructions, Atomic can be slower for RMW operations. [boqun: Move Flag into atomic/predefine.rs] Signed-off-by: FUJITA Tomonori Reviewed-by: Gary Guo Reviewed-by: Alice Ryhl Signed-off-by: Boqun Feng Link: https://patch.msgid.link/20260115021230.3297420-2-fujita.tomonori@gmail.com --- rust/kernel/sync/atomic.rs | 1 + rust/kernel/sync/atomic/predefine.rs | 71 ++++++++++++++++++++++++++++ 2 files changed, 72 insertions(+) diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs index 4aebeacb961a..915c625a8bc0 100644 --- a/rust/kernel/sync/atomic.rs +++ b/rust/kernel/sync/atomic.rs @@ -22,6 +22,7 @@ pub use internal::AtomicImpl; pub use ordering::{Acquire, Full, Relaxed, Release}; +pub use predefine::Flag; pub(crate) use internal::{AtomicArithmeticOps, AtomicBasicOps, AtomicExchangeOps}; diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs index 42067c6a266c..261e6d7f5341 100644 --- a/rust/kernel/sync/atomic/predefine.rs +++ b/rust/kernel/sync/atomic/predefine.rs @@ -122,6 +122,77 @@ fn rhs_into_delta(rhs: usize) -> isize_atomic_repr { } } +/// An atomic flag type intended to be backed by a performance-optimal integer type. +/// +/// The backing integer type is an implementation detail; it may vary by architecture and change +/// in the future. +/// +/// [`Atomic`] is generally preferable to [`Atomic`] when you need read-modify-write +/// (RMW) operations (e.g. [`Atomic::xchg()`]/[`Atomic::cmpxchg()`]) or when [`Atomic`] does +/// not save memory due to padding. On some architectures that do not support byte-sized atomic +/// RMW operations, RMW operations on [`Atomic`] are slower. +/// +/// If you only use [`Atomic::load()`]/[`Atomic::store()`], either [`Atomic`] or +/// [`Atomic`] is fine. +/// +/// # Examples +/// +/// ``` +/// use kernel::sync::atomic::{Atomic, Flag, Relaxed}; +/// +/// let flag = Atomic::new(Flag::Clear); +/// assert_eq!(Flag::Clear, flag.load(Relaxed)); +/// flag.store(Flag::Set, Relaxed); +/// assert_eq!(Flag::Set, flag.load(Relaxed)); +/// ``` +#[derive(Clone, Copy, PartialEq, Eq)] +#[cfg_attr(any(CONFIG_X86_64, CONFIG_UML, CONFIG_ARM, CONFIG_ARM64), repr(i8))] +#[cfg_attr( + not(any(CONFIG_X86_64, CONFIG_UML, CONFIG_ARM, CONFIG_ARM64)), + repr(i32) +)] +pub enum Flag { + /// The flag is clear. + Clear = 0, + /// The flag is set. + Set = 1, +} + +// SAFETY: `Flag` and `Repr` have the same size and alignment, and `Flag` is round-trip +// transmutable to the selected representation (`i8` or `i32`). +unsafe impl super::AtomicType for Flag { + #[cfg(any(CONFIG_X86_64, CONFIG_UML, CONFIG_ARM, CONFIG_ARM64))] + type Repr = i8; + #[cfg(not(any(CONFIG_X86_64, CONFIG_UML, CONFIG_ARM, CONFIG_ARM64)))] + type Repr = i32; +} + +impl Flag { + /// Creates a new [`Flag`] from a [`bool`]. + #[inline(always)] + pub const fn new(b: bool) -> Self { + if b { + Flag::Set + } else { + Flag::Clear + } + } +} + +impl From for bool { + #[inline(always)] + fn from(f: Flag) -> Self { + f == Flag::Set + } +} + +impl From for Flag { + #[inline(always)] + fn from(b: bool) -> Self { + Flag::new(b) + } +} + use crate::macros::kunit_tests; #[kunit_tests(rust_atomics)] -- 2.51.0