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use std::cell::UnsafeCell; use std::fmt; use std::ops::{Deref, DerefMut}; use std::panic::{RefUnwindSafe, UnwindSafe}; use std::sync::atomic::{AtomicU8, Ordering}; use crate::misc::{PhantomMutex, PhantomMutexGuard}; use crate::result::{LockResult, PoisonError, TryLockError, TryLockResult}; /// A mutual exclusion primitive useful for protecting shared data. /// /// It behaves like std::sync::Mutex except for using spinlock. /// What is more, the constructor is a const function; i.e. it is possible to declare /// static Mutex<T> variable as long as the inner data can be built statically. /// /// This mutex will block threads waiting for the lock to become available. The /// mutex can also be statically initialized or created via a [`new`] /// constructor. Each mutex has a type parameter which represents the data that /// it is protecting. The data can only be accessed through the RAII guards /// returned from [`lock`] and [`try_lock`], which guarantees that the data is only /// ever accessed when the mutex is locked. /// /// # Poisoning /// /// The mutexes in this module implement a strategy called "poisoning" where a /// mutex is considered poisoned whenever a thread panics while holding the /// mutex. Once a mutex is poisoned, all other threads are unable to access the /// data by default as it is likely tainted. /// /// For a mutex, this means that the [`lock`] and [`try_lock`] methods return a /// `Result` which indicates whether a mutex has been poisoned or not. Most /// usage of a mutex will simply `unwrap()` these results, propagating panics /// among threads to ensure that a possibly invalid invariant is not witnessed. /// /// A poisoned mutex, however, does not prevent all access to the underlying /// data. The [`PoisonError`] type has an `into_inner` method which will return /// the guard that would have otherwise been returned on a successful lock. This /// allows access to the data, despite the lock being poisoned. /// /// [`new`]: #method.new /// [`lock`]: #method.lock /// [`try_lock`]: #method.try_lock /// [`PoisonError`]: type.PoisonError.html /// /// # Examples /// /// Protect a variable (non-atomically) and update it in worker threads. /// /// ``` /// use spin_sync::Mutex; /// use std::thread; /// /// const WORKER_NUM: usize = 10; /// /// // We can declare static Mutex<usize> variable because Mutex::new is const. /// static MUTEX: Mutex<usize> = Mutex::new(0); /// /// let mut handles = Vec::with_capacity(WORKER_NUM); /// /// // Create worker threads to inclement the value by 1. /// for _ in 0..WORKER_NUM { /// let handle = thread::spawn(move || { /// let mut num = MUTEX.lock().unwrap(); /// *num += 1; /// }); /// /// handles.push(handle); /// } /// /// // Wait for the all worker threads are finished. /// for handle in handles { /// handle.join().unwrap(); /// } /// /// // Make sure the value is incremented by the worker count. /// let num = MUTEX.lock().unwrap(); /// assert_eq!(WORKER_NUM, *num); /// ``` /// /// To recover from a poisoned mutex: /// /// ``` /// use spin_sync::Mutex; /// use std::sync::Arc; /// use std::thread; /// /// // Like std::sync::Mutex, it can be declare as local variable, of course. /// let mutex = Arc::new(Mutex::new(0)); /// let c_mutex = mutex.clone(); /// /// let _ = thread::spawn(move || -> () { /// // This thread will acquire the mutex first, unwrapping the result of /// // `lock` because the lock has not been poisoned. /// let _guard = c_mutex.lock().unwrap(); /// /// // This panic while holding the lock (`_guard` is in scope) will poison /// // the mutex. /// panic!(); /// }).join(); /// /// // Here, the mutex has been poisoned. /// assert_eq!(true, mutex.is_poisoned()); /// /// // The returned result can be pattern matched on to return the underlying /// // guard on both branches. /// let mut guard = match mutex.lock() { /// Ok(guard) => guard, /// Err(poisoned) => poisoned.into_inner(), /// }; /// /// *guard += 1; /// assert_eq!(1, *guard); /// ``` pub struct Mutex<T: ?Sized> { lock: AtomicU8, _phantom: PhantomMutex<T>, data: UnsafeCell<T>, } impl<T> Mutex<T> { /// Creates a new mutex in an unlocked state ready for use. /// /// unlike to `std::sync::Mutex::new`, this is a const function. /// It can be use for static variable. /// /// # Examples /// /// Declare as a static variable. /// /// ``` /// use spin_sync::Mutex; /// /// static MUTEX: Mutex<i32> = Mutex::new(0); /// ``` /// /// Declare as a local variable. /// /// ``` /// use spin_sync::Mutex; /// /// let mutex = Mutex::new(0); /// ``` pub const fn new(t: T) -> Self { Mutex { lock: AtomicU8::new(INIT), data: UnsafeCell::new(t), _phantom: PhantomMutex {}, } } /// Consumes this mutex and returns the underlying data. /// /// Note that this method won't acquire any lock because we know there is /// no other references to `self`. /// /// # Errors /// /// If another user panicked while holding this mutex, this call wraps /// the result in an error and returns it. /// /// # Examples /// /// ``` /// use spin_sync::Mutex; /// /// let mutex = Mutex::new(0); /// assert_eq!(0, mutex.into_inner().unwrap()); /// ``` pub fn into_inner(self) -> LockResult<T> { let is_err = self.is_poisoned(); let data = self.data.into_inner(); if is_err { Err(PoisonError::new(data)) } else { Ok(data) } } } impl<T: ?Sized> Mutex<T> { /// Blocks the current thread until acquiring the lock, and returns an RAII guard object. /// /// The actual flow will be as follows. /// /// 1. User calls this method. /// 1. Blocks until this thread acquires the exclusive lock. /// 1. Creates an RAII guard object. /// 1. Wraps the guard in `Result` and returns it. If this mutex has been /// poisoned, it is wrapped in an `Err`; otherwise wrapped in a `Ok`. /// 1. User accesses to the underlying data through the returned guard. /// (No other thread can access to the data then.) /// 1. The guard is dropped (falls out of scope) and the lock is released. /// /// # Errors /// /// If another user panicked while holding this mutex, this method call wraps /// the guard in an error and returns it. /// /// # Examples /// /// ``` /// use spin_sync::Mutex; /// /// let mutex = Mutex::new(0); /// /// let mut guard = mutex.lock().unwrap(); /// assert_eq!(0, *guard); /// /// *guard += 1; /// assert_eq!(1, *guard); /// /// assert_eq!(true, mutex.try_lock().is_err()); /// ``` pub fn lock(&self) -> LockResult<MutexGuard<T>> { loop { match self.do_try_lock() { s if is_locked(s) => std::thread::yield_now(), s if is_poisoned(s) => return Err(PoisonError::new(MutexGuard::new(self))), _ => return Ok(MutexGuard::new(self)), } } } /// Attempts to acquire this lock and returns an RAII guard object if succeeded. /// /// Behaves like [`lock`] except for this method returns an error immediately if another /// user is holding the lock. /// /// This method does not block. /// /// The actual flow will be as follows. /// /// 1. User calls this method. /// 1. Tries to acquire the lock. If failed (i.e. if the lock is being held,) /// returns an error immediately and this flow is finished here. /// 1. Creates an RAII guard object. /// 1. Wrapps the guard in `Result` and returns it. If this mutex has been /// poisoned, it is wrapped in an `Err`; otherwise wrapped in an `Ok`. /// 1. User accesses to the underlying data through the returned guard. /// (No other thread can access to the data then.) /// 1. The guard is dropped (falls out of scope) and the lock is released. /// /// # Errors /// /// - If another user is holding this mutex, [`TryLockError::WouldBlock`] is returned. /// - If this function call succeeded to acquire the lock, and if another /// user panicked while holding this mutex, [`TryLockError::Poisoned`] is returned. /// /// [`lock`]: #method.lock /// [`TryLockError::WouldBlock`]: type.TryLockError.html /// [`TryLockError::Poisoned`]: type.TryLockError.html /// /// # Examples /// /// ``` /// use spin_sync::Mutex; /// /// let mutex = Mutex::new(0); /// /// // try_lock() fails while another guard is. /// // It doesn't cause a deadlock. /// { /// let _guard = mutex.lock().unwrap(); /// assert_eq!(true, mutex.try_lock().is_err()); /// } /// /// // try_lock() behaves like lock() if no other guard is. /// { /// let mut guard = mutex.try_lock().unwrap(); /// assert_eq!(true, mutex.try_lock().is_err()); /// *guard += 1; /// } /// /// let guard = mutex.try_lock().unwrap(); /// assert_eq!(1, *guard); /// ``` pub fn try_lock(&self) -> TryLockResult<MutexGuard<T>> { match self.do_try_lock() { s if is_locked(s) => Err(TryLockError::WouldBlock), s if is_poisoned(s) => Err(TryLockError::Poisoned(PoisonError::new(MutexGuard::new( self, )))), _ => Ok(MutexGuard::new(self)), } } /// Tries to acquire lock and returns the lock status before updated. fn do_try_lock(&self) -> LockStatus { // Assume neither poisoned nor locked at first. let mut expected = INIT; loop { let desired = acquire_lock(expected); match self .lock .compare_and_swap(expected, desired, Ordering::Acquire) { s if s == expected => return s, // Succeeded s if is_locked(s) => return s, // Another user is holding the lock. s => expected = s, // Assumption was wrong. Try again. } } } /// Determines whether the mutex is poisoned or not. /// /// # Warnings /// /// This function won't acquire any lock. If another thread is active, /// the mutex can become poisoned at any time. You should not trust a `false` /// value for program correctness without additional synchronization. /// /// This behavior is same to `std::sync::Mutex::is_poisoned()`. /// /// # Examples /// /// ``` /// use spin_sync::Mutex; /// use std::sync::Arc; /// use std::thread; /// /// let mutex = Arc::new(Mutex::new(0)); /// assert_eq!(false, mutex.is_poisoned()); /// /// // Panic and poison the mutex. /// { /// let mutex = mutex.clone(); /// /// let _ = thread::spawn(move || { /// // This panic while holding the lock (`_guard` is in scope) will poison /// // the mutex. /// let _guard = mutex.lock().unwrap(); /// panic!("Poison here"); /// }).join(); /// } /// /// assert_eq!(true, mutex.is_poisoned()); /// ``` pub fn is_poisoned(&self) -> bool { // Don't acquire any lock; otherwise, this function will cause // a deadlock if the caller thread is holding the lock. let status = self.lock.load(Ordering::Relaxed); return is_poisoned(status); } /// Returns a mutable reference to the underlying data. /// /// Note that this method won't acquire any lock because we know there is /// no other references to `self`. /// /// # Errors /// /// If another user panicked while holding this mutex, this method call /// wraps the result in an error and returns it. /// /// # Examples /// /// ``` /// use spin_sync::Mutex; /// /// let mut mutex = Mutex::new(0); /// *mutex.get_mut().unwrap() = 10; /// assert_eq!(10, *mutex.lock().unwrap()); /// ``` pub fn get_mut(&mut self) -> LockResult<&mut T> { // There is no other references to `self` because the argument is // a mutable reference. // No lock is required. let data = unsafe { &mut *self.data.get() }; if self.is_poisoned() { Err(PoisonError::new(data)) } else { Ok(data) } } } impl<T: ?Sized + fmt::Debug> fmt::Debug for Mutex<T> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { match self.try_lock() { Ok(guard) => f.debug_struct("Mutex").field("data", &&*guard).finish(), Err(TryLockError::Poisoned(err)) => f .debug_struct("Mutex") .field("data", &&**err.get_ref()) .finish(), Err(TryLockError::WouldBlock) => { struct LockedPlaceholder; impl fmt::Debug for LockedPlaceholder { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { f.write_str("<locked>") } } f.debug_struct("Mutex") .field("data", &LockedPlaceholder) .finish() } } } } impl<T> From<T> for Mutex<T> { fn from(t: T) -> Self { Mutex::new(t) } } impl<T: ?Sized + Default> Default for Mutex<T> { fn default() -> Self { Mutex::new(T::default()) } } /// An RAII implementation of a "scoped lock" of a mutex. /// /// When this structure is dropped (falls out of scope), the lock will be released. /// /// The data protected by the mutex can be accessed through this guard via its /// `Deref` and `DerefMut` implementations. /// /// This structure is created by [`lock`] and [`try_lock`] methods on /// [`Mutex`]. /// /// [`lock`]: struct.Mutex.html#method.lock /// [`try_lock`]: struct.Mutex.html#method.try_lock /// [`Mutex`]: struct.Mutex.html #[must_use = "if unused the Mutex will immediately unlock"] pub struct MutexGuard<'a, T: ?Sized + 'a> { mutex: &'a Mutex<T>, _phantom: PhantomMutexGuard<'a, T>, // To implement !Send. } impl<'a, T: ?Sized> MutexGuard<'a, T> { fn new(mutex: &'a Mutex<T>) -> Self { Self { mutex, _phantom: Default::default(), } } } impl<T: ?Sized> Drop for MutexGuard<'_, T> { fn drop(&mut self) { let old_status = self.mutex.lock.load(Ordering::Relaxed); debug_assert!(is_locked(old_status)); let mut new_status = release_lock(old_status); if std::thread::panicking() { new_status = set_poison_flag(new_status); } self.mutex.lock.store(new_status, Ordering::Release); } } impl<T: ?Sized> Deref for MutexGuard<'_, T> { type Target = T; fn deref(&self) -> &Self::Target { unsafe { &*self.mutex.data.get() } } } impl<T: ?Sized> DerefMut for MutexGuard<'_, T> { fn deref_mut(&mut self) -> &mut Self::Target { unsafe { &mut *self.mutex.data.get() } } } impl<T: ?Sized + fmt::Debug> fmt::Debug for MutexGuard<'_, T> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { fmt::Debug::fmt(&**self, f) } } impl<T: ?Sized + fmt::Display> fmt::Display for MutexGuard<'_, T> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { fmt::Display::fmt(&**self, f) } } // // Marker Traits // impl<T: ?Sized> UnwindSafe for Mutex<T> {} impl<T: ?Sized> RefUnwindSafe for Mutex<T> {} unsafe impl<T: ?Sized + Send> Send for Mutex<T> {} unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {} unsafe impl<T: ?Sized + Sync> Sync for MutexGuard<'_, T> {} // // Constants to represent lock state // type LockStatus = u8; const INIT: LockStatus = 0; const LOCK_FLAG: LockStatus = 0x01; const POISON_FLAG: LockStatus = 0x02; const NOT_USED_MASK: LockStatus = 0xfc; #[inline] #[must_use] fn is_locked(s: LockStatus) -> bool { debug_assert_eq!(0, s & NOT_USED_MASK); (s & LOCK_FLAG) != 0 } #[inline] #[must_use] fn acquire_lock(s: LockStatus) -> LockStatus { debug_assert_eq!(false, is_locked(s)); s | LOCK_FLAG } #[inline] #[must_use] fn release_lock(s: LockStatus) -> LockStatus { debug_assert_eq!(true, is_locked(s)); s & !(LOCK_FLAG) } #[inline] #[must_use] fn is_poisoned(s: LockStatus) -> bool { debug_assert_eq!(0, s & NOT_USED_MASK); (s & POISON_FLAG) != 0 } #[inline] #[must_use] fn set_poison_flag(s: LockStatus) -> LockStatus { debug_assert_eq!(0, s & NOT_USED_MASK); s | POISON_FLAG }