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Tue, 27 Jan 2026 11:34:59 -0500 (EST) Date: Tue, 27 Jan 2026 08:34:58 -0800 From: Boqun Feng To: Gary Guo Cc: FUJITA Tomonori , ojeda@kernel.org, peterz@infradead.org, will@kernel.org, a.hindborg@kernel.org, aliceryhl@google.com, bjorn3_gh@protonmail.com, dakr@kernel.org, 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: On Tue, Jan 27, 2026 at 04:10:35PM +0000, Gary Guo wrote: > On Tue Jan 27, 2026 at 3:59 PM GMT, Boqun Feng wrote: > > On Tue, Jan 27, 2026 at 03:42:42PM +0000, Gary Guo wrote: > >> On Tue Jan 27, 2026 at 3:35 PM GMT, Boqun Feng wrote: > >> > [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 > >> > >> I think we really need special handling for endianness for this one single > >> function, so doing all the extra stuff feels really unnecessary. > >> > > > > First, this one single function changes the design actually, previously > > you can even implement a Flag as: > > > > enum Flag { > > Clear = 6, > > Set = 7, > > } > > > > and it'll work, that is as long as `Flag` behaves like a bool, it's > > fine. But now this function implies there is actually a bool in `Flag`, > > which is kinda totally different. > > > > Besides, by using the current implement, we set an example about "how to > > do a byte offset in an i32 for different endians", and then if anyone > > wanted to do something similar, very likely they would copy-paste and > > modify what we have here. The potential tech debts are significant. So I > > would like to do it in a right way ("right" is probably subjective, but > > it comes from someone who needs to live with the code as a maintainer > > ;-) and I'm happy to switch to a better way if necessary). > > I think what Fujita has is more "proper". Your approach still have the issue of > requiring a specific ordering of the fields. If this is messed up, then the > entire thing is broken. I.e. the safety proof of `get_mut` depends on the fields > being ordered correctly in `FlagInner`. > > If you want to go down this route then I would just scrap `enum Flag` all > together and always define it as struct, with an internal `bool` + 3 bytes of > zero padding. This way we don't even need unsafe for `get_mut`. > Hmm.. so like: const PAD_SIZE: usize = <3 or 0 depending on ARCHs> /// # Invariants /// `pad` has to be all zero. struct Flag { bool_field: bool, pad: [i8; PAD_SIZE], } impl Flag { pub const fn set() -> Flag { Self { true, pad: [0; PAD_SIZE] } } pub const fn clear() -> Flag { Self { false, pad: [0; PAD_SIZE] } } } ? Yes, I think it's better ;-) Also, now given that `AtomicFlag` behaves exactly like a `Atomic`, should we do: /// `AtomicFlag` documentation here. #[cfg()] type AtomicFlag = Atomic; #[cfg()] struct AtomicFlag(Atomic); // `Flag` doesn't even need to be public. #[cfg()] struct BooleanFlag { ... } (I renamed `Flag` -> `BooleanFlag`) Thoughts? I don't think there is any extra benefit of exposing `Atomic`. Regards, Boqun > Best, > Gary > > > > Hope this can explain why I want to do this right now. > > > > Regards, > > Boqun > > > >> I prefer Fujita's current version. > >> > >> Best, > >> Gary > >> > >> > > >> >> + > >> >> + // 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 > >> >> > >> >