# spin.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/spin-0.7.1/src/mutex/spin.rs`.
Keywords: rust, rustlang, rust-lang

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    use core::{
        cell::UnsafeCell,
        fmt,
        ops::{Deref, DerefMut},
        sync::atomic::{AtomicBool, Ordering},
    };

    /// A [spin lock](https://en.m.wikipedia.org/wiki/Spinlock) providing mutually exclusive access to data.
    ///
    /// # Example
    ///
    /// ```
    /// use spin;
    ///
    /// let lock = spin::mutex::SpinMutex::new(0);
    ///
    /// // Modify the data
    /// *lock.lock() = 2;
    ///
    /// // Read the data
    /// let answer = *lock.lock();
    /// assert_eq!(answer, 2);
    /// ```
    ///
    /// # Thread safety example
    ///
    /// ```
    /// use spin;
    /// use std::sync::{Arc, Barrier};
    ///
    /// let thread_count = 1000;
    /// let spin_mutex = Arc::new(spin::mutex::SpinMutex::new(0));
    ///
    /// // We use a barrier to ensure the readout happens after all writing
    /// let barrier = Arc::new(Barrier::new(thread_count + 1));
    ///
    /// for _ in (0..thread_count) {
    ///     let my_barrier = barrier.clone();
    ///     let my_lock = spin_mutex.clone();
    ///     std::thread::spawn(move || {
    ///         let mut guard = my_lock.lock();
    ///         *guard += 1;
    ///
    ///         // Release the lock to prevent a deadlock
    ///         drop(guard);
    ///         my_barrier.wait();
    ///     });
    /// }
    ///
    /// barrier.wait();
    ///
    /// let answer = { *spin_mutex.lock() };
    /// assert_eq!(answer, thread_count);
    /// ```
    pub struct SpinMutex<T: ?Sized> {
        pub(crate) lock: AtomicBool,
        data: UnsafeCell<T>,
    }

    /// A guard that provides mutable data access.
    ///
    /// When the guard falls out of scope it will release the lock.
    pub struct SpinMutexGuard<'a, T: ?Sized + 'a> {
        lock: &'a AtomicBool,
        data: &'a mut T,
    }

    // Same unsafe impls as `std::sync::Mutex`
    unsafe impl<T: ?Sized + Send> Sync for SpinMutex<T> {}
    unsafe impl<T: ?Sized + Send> Send for SpinMutex<T> {}

    impl<T> SpinMutex<T> {
        /// Creates a new [`SpinMutex`] wrapping the supplied data.
        ///
        /// # Example
        ///
        /// ```
        /// use spin::mutex::SpinMutex;
        ///
        /// static MUTEX: SpinMutex<()> = SpinMutex::new(());
        ///
        /// fn demo() {
        ///     let lock = MUTEX.lock();
        ///     // do something with lock
        ///     drop(lock);
        /// }
        /// ```
        #[inline(always)]
        pub const fn new(user_data: T) -> SpinMutex<T> {
            SpinMutex {
                lock: AtomicBool::new(false),
                data: UnsafeCell::new(user_data),
            }
        }

        /// Consumes this [`SpinMutex`] and unwraps the underlying data.
        ///
        /// # Example
        ///
        /// ```
        /// let lock = spin::mutex::SpinMutex::new(42);
        /// assert_eq!(42, lock.into_inner());
        /// ```
        #[inline(always)]
        pub fn into_inner(self) -> T {
            // We know statically that there are no outstanding references to
            // `self` so there's no need to lock.
            let SpinMutex { data, .. } = self;
            data.into_inner()
        }
    }

    impl<T: ?Sized> SpinMutex<T> {
        /// Returns `true` if the lock is currently held.
        ///
        /// # Safety
        ///
        /// This function provides no synchronization guarantees and so its result should be considered 'out of date'
        /// the instant it is called. Do not use it for synchronization purposes. However, it may be useful as a heuristic.
        #[inline(always)]
        pub fn is_locked(&self) -> bool {
            self.lock.load(Ordering::Relaxed)
        }

        /// Locks the [`SpinMutex`] and returns a guard that permits access to the inner data.
        ///
        /// The returned value may be dereferenced for data access
        /// and the lock will be dropped when the guard falls out of scope.
        ///
        /// ```
        /// let lock = spin::mutex::SpinMutex::new(0);
        /// {
        ///     let mut data = lock.lock();
        ///     // The lock is now locked and the data can be accessed
        ///     *data += 1;
        ///     // The lock is implicitly dropped at the end of the scope
        /// }
        /// ```
        #[inline(always)]
        pub fn lock(&self) -> SpinMutexGuard<T> {
            // Can fail to lock even if the spinlock is not locked. May be more efficient than `try_lock`
            // when called in a loop.
            while self.lock.compare_exchange_weak(false, true, Ordering::Acquire, Ordering::Relaxed).is_err() {
                // Wait until the lock looks unlocked before retrying
                while self.is_locked() {
                    crate::relax();
                }
            }

            SpinMutexGuard {
                lock: &self.lock,
                data: unsafe { &mut *self.data.get() },
            }
        }

        /// Force unlock this [`SpinMutex`].
        ///
        /// # Safety
        ///
        /// This is *extremely* unsafe if the lock is not held by the current
        /// thread. However, this can be useful in some instances for exposing the
        /// lock to FFI that doesn't know how to deal with RAII.
        #[inline(always)]
        pub unsafe fn force_unlock(&self) {
            self.lock.store(false, Ordering::Release);
        }

        /// Try to lock this [`SpinMutex`], returning a lock guard if successful.
        ///
        /// # Example
        ///
        /// ```
        /// let lock = spin::mutex::SpinMutex::new(42);
        ///
        /// let maybe_guard = lock.try_lock();
        /// assert!(maybe_guard.is_some());
        ///
        /// // `maybe_guard` is still held, so the second call fails
        /// let maybe_guard2 = lock.try_lock();
        /// assert!(maybe_guard2.is_none());
        /// ```
        #[inline(always)]
        pub fn try_lock(&self) -> Option<SpinMutexGuard<T>> {
            // The reason for using a strong compare_exchange is explained here:
            // https://github.com/Amanieu/parking_lot/pull/207#issuecomment-575869107
            if self.lock.compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed).is_ok() {
                Some(SpinMutexGuard {
                    lock: &self.lock,
                    data: unsafe { &mut *self.data.get() },
                })
            } else {
                None
            }
        }

        /// Returns a mutable reference to the underlying data.
        ///
        /// Since this call borrows the [`SpinMutex`] mutably, and a mutable reference is guaranteed to be exclusive in
        /// Rust, no actual locking needs to take place -- the mutable borrow statically guarantees no locks exist. As
        /// such, this is a 'zero-cost' operation.
        ///
        /// # Example
        ///
        /// ```
        /// let mut lock = spin::mutex::SpinMutex::new(0);
        /// *lock.get_mut() = 10;
        /// assert_eq!(*lock.lock(), 10);
        /// ```
        #[inline(always)]
        pub fn get_mut(&mut self) -> &mut T {
            // We know statically that there are no other references to `self`, so
            // there's no need to lock the inner mutex.
            unsafe { &mut *self.data.get() }
        }
    }

    impl<T: ?Sized + fmt::Debug> fmt::Debug for SpinMutex<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match self.try_lock() {
                Some(guard) => write!(f, "Mutex {{ data: ")
                    .and_then(|()| (&*guard).fmt(f))
                    .and_then(|()| write!(f, "}}")),
                None => write!(f, "Mutex {{ <locked> }}"),
            }
        }
    }

    impl<T: ?Sized + Default> Default for SpinMutex<T> {
        fn default() -> SpinMutex<T> {
            SpinMutex::new(Default::default())
        }
    }

    impl<T> From<T> for SpinMutex<T> {
        fn from(data: T) -> Self {
            Self::new(data)
        }
    }

    impl<'a, T: ?Sized> SpinMutexGuard<'a, T> {
        /// Leak the lock guard, yielding a mutable reference to the underlying data.
        ///
        /// Note that this function will permanently lock the original [`SpinMutex`].
        ///
        /// ```
        /// let mylock = spin::mutex::SpinMutex::new(0);
        ///
        /// let data: &mut i32 = spin::mutex::SpinMutexGuard::leak(mylock.lock());
        ///
        /// *data = 1;
        /// assert_eq!(*data, 1);
        /// ```
        #[inline(always)]
        pub fn leak(this: Self) -> &'a mut T {
            let data = this.data as *mut _; // Keep it in pointer form temporarily to avoid double-aliasing
            core::mem::forget(this);
            unsafe { &mut *data }
        }
    }

    impl<'a, T: ?Sized + fmt::Debug> fmt::Debug for SpinMutexGuard<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Debug::fmt(&**self, f)
        }
    }

    impl<'a, T: ?Sized + fmt::Display> fmt::Display for SpinMutexGuard<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Display::fmt(&**self, f)
        }
    }

    impl<'a, T: ?Sized> Deref for SpinMutexGuard<'a, T> {
        type Target = T;
        fn deref(&self) -> &T {
            self.data
        }
    }

    impl<'a, T: ?Sized> DerefMut for SpinMutexGuard<'a, T> {
        fn deref_mut(&mut self) -> &mut T {
            self.data
        }
    }

    impl<'a, T: ?Sized> Drop for SpinMutexGuard<'a, T> {
        /// The dropping of the MutexGuard will release the lock it was created from.
        fn drop(&mut self) {
            self.lock.store(false, Ordering::Release);
        }
    }

    #[cfg(feature = "lock_api1")]
    unsafe impl lock_api::RawMutex for SpinMutex<()> {
        type GuardMarker = lock_api::GuardSend;

        const INIT: Self = Self::new(());

        fn lock(&self) {
            // Prevent guard destructor running
            core::mem::forget(Self::lock(self));
        }

        fn try_lock(&self) -> bool {
            // Prevent guard destructor running
            Self::try_lock(self).map(core::mem::forget).is_some()
        }

        unsafe fn unlock(&self) {
            self.force_unlock();
        }

        fn is_locked(&self) -> bool {
            Self::is_locked(self)
        }
    }

    #[cfg(test)]
    mod tests {
        use std::prelude::v1::*;

        use std::sync::atomic::{AtomicUsize, Ordering};
        use std::sync::mpsc::channel;
        use std::sync::Arc;
        use std::thread;

        use super::*;

        #[derive(Eq, PartialEq, Debug)]
        struct NonCopy(i32);

        #[test]
        fn smoke() {
            let m = SpinMutex::new(());
            drop(m.lock());
            drop(m.lock());
        }

        #[test]
        fn lots_and_lots() {
            static M: SpinMutex<()> = SpinMutex::new(());
            static mut CNT: u32 = 0;
            const J: u32 = 1000;
            const K: u32 = 3;

            fn inc() {
                for _ in 0..J {
                    unsafe {
                        let _g = M.lock();
                        CNT += 1;
                    }
                }
            }

            let (tx, rx) = channel();
            for _ in 0..K {
                let tx2 = tx.clone();
                thread::spawn(move || {
                    inc();
                    tx2.send(()).unwrap();
                });
                let tx2 = tx.clone();
                thread::spawn(move || {
                    inc();
                    tx2.send(()).unwrap();
                });
            }

            drop(tx);
            for _ in 0..2 * K {
                rx.recv().unwrap();
            }
            assert_eq!(unsafe { CNT }, J * K * 2);
        }

        #[test]
        fn try_lock() {
            let mutex = SpinMutex::new(42);

            // First lock succeeds
            let a = mutex.try_lock();
            assert_eq!(a.as_ref().map(|r| **r), Some(42));

            // Additional lock failes
            let b = mutex.try_lock();
            assert!(b.is_none());

            // After dropping lock, it succeeds again
            ::core::mem::drop(a);
            let c = mutex.try_lock();
            assert_eq!(c.as_ref().map(|r| **r), Some(42));
        }

        #[test]
        fn test_into_inner() {
            let m = SpinMutex::new(NonCopy(10));
            assert_eq!(m.into_inner(), NonCopy(10));
        }

        #[test]
        fn test_into_inner_drop() {
            struct Foo(Arc<AtomicUsize>);
            impl Drop for Foo {
                fn drop(&mut self) {
                    self.0.fetch_add(1, Ordering::SeqCst);
                }
            }
            let num_drops = Arc::new(AtomicUsize::new(0));
            let m = SpinMutex::new(Foo(num_drops.clone()));
            assert_eq!(num_drops.load(Ordering::SeqCst), 0);
            {
                let _inner = m.into_inner();
                assert_eq!(num_drops.load(Ordering::SeqCst), 0);
            }
            assert_eq!(num_drops.load(Ordering::SeqCst), 1);
        }

        #[test]
        fn test_mutex_arc_nested() {
            // Tests nested mutexes and access
            // to underlying data.
            let arc = Arc::new(SpinMutex::new(1));
            let arc2 = Arc::new(SpinMutex::new(arc));
            let (tx, rx) = channel();
            let _t = thread::spawn(move || {
                let lock = arc2.lock();
                let lock2 = lock.lock();
                assert_eq!(*lock2, 1);
                tx.send(()).unwrap();
            });
            rx.recv().unwrap();
        }

        #[test]
        fn test_mutex_arc_access_in_unwind() {
            let arc = Arc::new(SpinMutex::new(1));
            let arc2 = arc.clone();
            let _ = thread::spawn(move || -> () {
                struct Unwinder {
                    i: Arc<SpinMutex<i32>>,
                }
                impl Drop for Unwinder {
                    fn drop(&mut self) {
                        *self.i.lock() += 1;
                    }
                }
                let _u = Unwinder { i: arc2 };
                panic!();
            })
            .join();
            let lock = arc.lock();
            assert_eq!(*lock, 2);
        }

        #[test]
        fn test_mutex_unsized() {
            let mutex: &SpinMutex<[i32]> = &SpinMutex::new([1, 2, 3]);
            {
                let b = &mut *mutex.lock();
                b[0] = 4;
                b[2] = 5;
            }
            let comp: &[i32] = &[4, 2, 5];
            assert_eq!(&*mutex.lock(), comp);
        }

        #[test]
        fn test_mutex_force_lock() {
            let lock = SpinMutex::new(());
            ::std::mem::forget(lock.lock());
            unsafe {
                lock.force_unlock();
            }
            assert!(lock.try_lock().is_some());
        }
    }