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Tue, 27 Jan 2026 10:35:35 -0500 (EST) Date: Tue, 27 Jan 2026 07:35:34 -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, FUJITA Tomonori Subject: Re: [PATCH v2 1/2] rust: sync: atomic: Add AtomicFlag::get_mut Message-ID: References: <20260127125300.3656544-1-tomo@aliasing.net> <20260127125300.3656544-2-tomo@aliasing.net> 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: <20260127125300.3656544-2-tomo@aliasing.net> [For some unknown reasons, I cannot send my reply via gmail hence reply via kernel.org account, I might switch from gmail later on] On Tue, Jan 27, 2026 at 09:52:59PM +0900, FUJITA Tomonori wrote: > From: FUJITA Tomonori > > AtomicFlag exposes a bool API, but it lacks a get_mut() equivalent to > Atomic::get_mut(). > > Also add kunit tests for AtomicFlag. > > Signed-off-by: FUJITA Tomonori > --- > rust/kernel/sync/atomic.rs | 20 ++++++++++++++++++++ > rust/kernel/sync/atomic/predefine.rs | 17 +++++++++++++++++ > 2 files changed, 37 insertions(+) > > diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs > index 6c46335bdb8c..b6c01d9f3a46 100644 > --- a/rust/kernel/sync/atomic.rs > +++ b/rust/kernel/sync/atomic.rs > @@ -591,6 +591,26 @@ pub fn store(&self, b: bool, o: Ordering) > self.0.store(b.into(), o) > } > > + /// Returns a mutable reference to the underlying flag as a `bool`. > + /// > + /// This is safe because the mutable reference of the atomic flag guarantees exclusive access. > + pub fn get_mut(&mut self) -> &mut bool { > + let byte_ptr = { > + let ptr = self.0.as_ptr().cast::(); > + let offset = if cfg!(target_endian = "big") { > + core::mem::size_of::() - 1 > + } else { > + 0 > + }; The idea is solid, but I want to avoid endian handling in the function, I would prefer a "struct declaration" solution like: #[cfg(target_endian = "big")] #[repr(align(4))] pub(super) struct FlagInner { _pad: [i8; 3], bool_field: bool, } #[cfg(target_endian = "little")] #[repr(align(4))] struct FlagInner { bool_field: bool, _pad: [i8; 3], } redefine `Flag` as `BoolFlag` #[repr(i32)] pub enum BoolFlag { Clear = 0, Set = 1, } and `Flag` becomes a union of `BoolFlag` and `FlagInner`: /// # Invariants /// `Flag` is either 0 or 1 in a i32 representation which implies /// that `inner` is always valid as long as `_pad` stays 0. pub union Flag { pub(super) inner: FlagInner, pub flag: BoolFlag, } // can static_assert that `Flag` and `BoolFlag` has the same // alignement and size. then impl AtomicFlag { pub fn get_mut(&mut self) -> &mut bool { let flag = self.0.get_mut(); // <- &mut Flag // INVARIANTS: flag.inner._pad cannot be modified via the // returned reference. // SAFETY: Per type invariants, `flag.inner.bool_field` is // always a valid bool. unsafe { &mut flag.inner.bool_field } } } Thoughts? Regards, Boqun > + > + // SAFETY: `ptr` is valid for `size_of::()` bytes; `offset` selects the LSB. > + unsafe { ptr.add(offset) } > + }; > + > + // SAFETY: The LSB holds `0`/`1` for `Flag::Clear/Set`, and `bool` is `i8`-sized/aligned. > + unsafe { &mut *byte_ptr.cast::() } > + } > + > /// Stores a value to the atomic flag and returns the previous value. > #[inline(always)] > pub fn xchg(&self, b: bool, o: Ordering) -> bool { > diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs > index 11bc67ab70a3..e413b9e9fe21 100644 > --- a/rust/kernel/sync/atomic/predefine.rs > +++ b/rust/kernel/sync/atomic/predefine.rs > @@ -351,6 +351,23 @@ fn atomic_bool_tests() { > assert_eq!(Ok(false), x.cmpxchg(false, true, Full)); > } > > + #[test] > + fn atomic_flag_tests() { > + let mut flag = AtomicFlag::new(false); > + > + assert_eq!(false, flag.load(Relaxed)); > + > + *flag.get_mut() = true; > + assert_eq!(true, flag.load(Relaxed)); > + > + assert_eq!(true, flag.xchg(false, Relaxed)); > + assert_eq!(false, flag.load(Relaxed)); > + > + *flag.get_mut() = true; > + assert_eq!(Ok(true), flag.cmpxchg(true, false, Full)); > + assert_eq!(false, flag.load(Relaxed)); > + } > + > #[test] > fn atomic_ptr_tests() { > let mut v = 42; > -- > 2.43.0 >