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// This implementation is based on: // https://github.com/Amanieu/parking_lot/tree/fa294cd677936bf365afa0497039953b10c722f5/lock_api // and // https://github.com/mvdnes/spin-rs/tree/7516c8037d3d15712ba4d8499ab075e97a19d778 use lock_api::{RawMutex, GuardSend}; use core::sync::atomic::{AtomicBool, Ordering, spin_loop_hint}; /// Provides mutual exclusion based on spinning on an `AtomicBool`. /// /// It's recommended to use this type either combination with [`lock_api::Mutex`] or /// through the [`Spinlock`] type. /// /// ## Example /// /// ```rust /// use lock_api::RawMutex; /// let lock = spinning_top::RawSpinlock::INIT; /// assert_eq!(lock.try_lock(), true); // lock it /// assert_eq!(lock.try_lock(), false); // can't be locked a second time /// lock.unlock(); // unlock it /// assert_eq!(lock.try_lock(), true); // now it can be locked again #[derive(Debug)] pub struct RawSpinlock { /// Whether the spinlock is locked. locked: AtomicBool, } unsafe impl RawMutex for RawSpinlock { const INIT: RawSpinlock = RawSpinlock { locked: AtomicBool::new(false), }; // A spinlock guard can be sent to another thread and unlocked there type GuardMarker = GuardSend; fn lock(&self) { while !self.try_lock() { // Wait until the lock looks unlocked before retrying // Code from https://github.com/mvdnes/spin-rs/commit/d3e60d19adbde8c8e9d3199c7c51e51ee5a20bf6 while self.locked.load(Ordering::Relaxed) { // Tell the CPU that we're inside a busy-wait loop spin_loop_hint(); } } } fn try_lock(&self) -> bool { self.locked.compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed).is_ok() } fn unlock(&self) { self.locked.store(false, Ordering::Release); } } /// A mutual exclusion (Mutex) type based on busy-waiting. /// /// Calling `lock` (or `try_lock`) on this type returns a [`SpinlockGuard`], which /// automatically frees the lock when it goes out of scope. /// /// ## Example /// /// ```rust /// use spinning_top::Spinlock; /// /// fn main() { /// // Wrap some data in a spinlock /// let data = String::from("Hello"); /// let spinlock = Spinlock::new(data); /// make_uppercase(&spinlock); // only pass a shared reference /// /// // We have ownership of the spinlock, so we can extract the data without locking /// // Note: this consumes the spinlock /// let data = spinlock.into_inner(); /// assert_eq!(data.as_str(), "HELLO"); /// } /// /// fn make_uppercase(spinlock: &Spinlock<String>) { /// // Lock the spinlock to get a mutable reference to the data /// let mut locked_data = spinlock.lock(); /// assert_eq!(locked_data.as_str(), "Hello"); /// locked_data.make_ascii_uppercase(); /// /// // the lock is automatically freed at the end of the scope /// } /// ``` /// /// ## Nightly Example /// /// On Rust nightly, the `new` function is a `const` function, which makes the /// `Spinlock` type usable in statics: /// /// ```rust,ignore /// use spinning_top::Spinlock; /// /// static DATA: Spinlock<u32> = Spinlock::new(0); /// /// fn main() { /// let mut data = DATA.lock(); /// *data += 1; /// assert_eq!(*data, 1); /// } /// ``` pub type Spinlock<T> = lock_api::Mutex<RawSpinlock, T>; /// A RAII guard that frees the spinlock when it goes out of scope. /// /// Allows access to the locked data through the [`core::ops::Deref`] and [`core::ops::DerefMut`] operations. /// /// ## Example /// /// ```rust /// use spinning_top::{Spinlock, SpinlockGuard}; /// /// let spinlock = Spinlock::new(Vec::new()); /// /// // begin a new scope /// { /// // lock the spinlock to create a `SpinlockGuard` /// let mut guard: SpinlockGuard<_> = spinlock.lock(); /// /// // guard can be used like a `&mut Vec` since it implements `DerefMut` /// guard.push(1); /// guard.push(2); /// assert_eq!(guard.len(), 2); /// } // guard is dropped -> frees the spinlock again /// /// // spinlock is unlocked again /// assert!(spinlock.try_lock().is_some()); pub type SpinlockGuard<'a, T> = lock_api::MutexGuard<'a, RawSpinlock, T>; #[cfg(test)] mod tests { use super::*; #[test] fn create_and_lock() { let spinlock = Spinlock::new(42); let data = spinlock.try_lock(); assert!(data.is_some()); assert_eq!(*data.unwrap(), 42); } #[test] fn mutual_exclusion() { let spinlock = Spinlock::new(1); let data = spinlock.try_lock(); assert!(data.is_some()); assert!(spinlock.try_lock().is_none()); assert!(spinlock.try_lock().is_none()); // still None core::mem::drop(data); assert!(spinlock.try_lock().is_some()); } #[test] fn three_locks() { let spinlock1 = Spinlock::new(1); let spinlock2 = Spinlock::new(2); let spinlock3 = Spinlock::new(3); let data1 = spinlock1.try_lock(); let data2 = spinlock2.try_lock(); let data3 = spinlock3.try_lock(); assert!(data1.is_some()); assert!(data2.is_some()); assert!(data3.is_some()); assert!(spinlock1.try_lock().is_none()); assert!(spinlock1.try_lock().is_none()); // still None assert!(spinlock2.try_lock().is_none()); assert!(spinlock3.try_lock().is_none()); core::mem::drop(data3); assert!(spinlock3.try_lock().is_some()); } }