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([2601:646:8700:dd30:5f3e:5ba7:e0ea:9a08]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-31a9cc5c8a9sm1874182a91.15.2025.07.03.02.36.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 03 Jul 2025 02:36:41 -0700 (PDT) Message-ID: <35e1fef4-b715-4827-a498-bdde9b58b51c@penguintechs.org> Date: Thu, 3 Jul 2025 02:36:39 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v14 1/7] rust: sync: add `OnceLock` To: Andreas Hindborg , Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= , Alice Ryhl , Masahiro Yamada , Nathan Chancellor , Luis Chamberlain , Danilo Krummrich , Benno Lossin , Nicolas Schier Cc: Trevor Gross , Adam Bratschi-Kaye , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kbuild@vger.kernel.org, Petr Pavlu , Sami Tolvanen , Daniel Gomez , Simona Vetter , Greg KH , Fiona Behrens , Daniel Almeida , linux-modules@vger.kernel.org References: <20250702-module-params-v3-v14-0-5b1cc32311af@kernel.org> <20250702-module-params-v3-v14-1-5b1cc32311af@kernel.org> Content-Language: en-US From: Wren Turkal In-Reply-To: <20250702-module-params-v3-v14-1-5b1cc32311af@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 7/2/25 6:18 AM, Andreas Hindborg wrote: > Introduce the `OnceLock` type, a container that can only be written once. > The container uses an internal atomic to synchronize writes to the internal > value. > > Signed-off-by: Andreas Hindborg > --- > rust/kernel/sync.rs | 1 + > rust/kernel/sync/once_lock.rs | 104 ++++++++++++++++++++++++++++++++++++++++++ > 2 files changed, 105 insertions(+) > > diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs > index c7c0e552bafe..f2ee07315091 100644 > --- a/rust/kernel/sync.rs > +++ b/rust/kernel/sync.rs > @@ -15,6 +15,7 @@ > mod condvar; > pub mod lock; > mod locked_by; > +pub mod once_lock; > pub mod poll; > pub mod rcu; > > diff --git a/rust/kernel/sync/once_lock.rs b/rust/kernel/sync/once_lock.rs > new file mode 100644 > index 000000000000..cd311bea3919 > --- /dev/null > +++ b/rust/kernel/sync/once_lock.rs > @@ -0,0 +1,104 @@ > +//! A container that can be initialized at most once. > + > +use super::atomic::ordering::Acquire; > +use super::atomic::ordering::Release; > +use super::atomic::Atomic; > +use kernel::types::Opaque; > + > +/// A container that can be populated at most once. Thread safe. > +/// > +/// Once the a [`OnceLock`] is populated, it remains populated by the same object for the > +/// lifetime `Self`. > +/// > +/// # Invariants > +/// > +/// `init` tracks the state of the container: > +/// > +/// - If the container is empty, `init` is `0`. > +/// - If the container is mutably accessed, `init` is `1`. > +/// - If the container is populated and ready for shared access, `init` is `2`. > +/// > +/// # Example > +/// > +/// ``` > +/// # use kernel::sync::once_lock::OnceLock; > +/// let value = OnceLock::new(); > +/// assert_eq!(None, value.as_ref()); > +/// > +/// let status = value.populate(42u8); > +/// assert_eq!(true, status); > +/// assert_eq!(Some(&42u8), value.as_ref()); > +/// assert_eq!(Some(42u8), value.copy()); > +/// > +/// let status = value.populate(101u8); > +/// assert_eq!(false, status); > +/// assert_eq!(Some(&42u8), value.as_ref()); > +/// assert_eq!(Some(42u8), value.copy()); > +/// ``` > +pub struct OnceLock { > + init: Atomic, > + value: Opaque, > +} This type looks very much like the Once type in rust's stdlib. I am wondering if the api could be changed to match that api. I know that this type is trying to provide a version subset of std::sync::OnceLock that doesn't allow resetting the type like these apis: * https://doc.rust-lang.org/std/sync/struct.OnceLock.html#method.get_mut * https://doc.rust-lang.org/std/sync/struct.OnceLock.html#method.take However, these methods can only be used on mut. See here for failing example: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=a78e51203c5b9555e3c151e162f0acab I think it might make more sense to match the api of the stdlib API and maybe only implement the methods you need. > + > +impl Default for OnceLock { > + fn default() -> Self { > + Self::new() > + } > +} Any reason not to use #[derive(Default)]? > + > +impl OnceLock { > + /// Create a new [`OnceLock`]. > + /// > + /// The returned instance will be empty. > + pub const fn new() -> Self { Like new in std OnceLock. Matches. Good. > + // INVARIANT: The container is empty and we set `init` to `0`. > + Self { > + value: Opaque::uninit(), > + init: Atomic::new(0), > + } > + } > + > + /// Get a reference to the contained object. > + /// > + /// Returns [`None`] if this [`OnceLock`] is empty. > + pub fn as_ref(&self) -> Option<&T> { Looks like the get method in the OnceLock. > + if self.init.load(Acquire) == 2 { > + // SAFETY: As determined by the load above, the object is ready for shared access. > + Some(unsafe { &*self.value.get() }) > + } else { > + None > + } > + } > + > + /// Populate the [`OnceLock`]. > + /// > + /// Returns `true` if the [`OnceLock`] was successfully populated. > + pub fn populate(&self, value: T) -> bool { Looks like set in OnceLock. Might also be worth implementing get_or_{try,}init, which get the value while initializing. > + // INVARIANT: We obtain exclusive access to the contained allocation and write 1 to > + // `init`. > + if let Ok(0) = self.init.cmpxchg(0, 1, Acquire) { > + // SAFETY: We obtained exclusive access to the contained object. > + unsafe { core::ptr::write(self.value.get(), value) }; > + // INVARIANT: We release our exclusive access and transition the object to shared > + // access. > + self.init.store(2, Release); > + true > + } else { > + false > + } > + } > +} > + > +impl OnceLock { > + /// Get a copy of the contained object. > + /// > + /// Returns [`None`] if the [`OnceLock`] is empty. > + pub fn copy(&self) -> Option { No equivalent in OnceLock. Similar to something like this: x.get().copied().unwrap(); // x is a OnceLock Example: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=f21068e55f73722544fb5ad341bce1c5 Maybe not specifically needed? > + if self.init.load(Acquire) == 2 { > + // SAFETY: As determined by the load above, the object is ready for shared access. > + Some(unsafe { *self.value.get() }) > + } else { > + None > + } > + } > +} > -- You're more amazing than you think!