library/std/src/sys/pal/sgx/waitqueue/spin_mutex.rs

//! Trivial spinlock-based implementation of `sync::Mutex`.
// FIXME: Perhaps use Intel TSX to avoid locking?

#[cfg(test)]
mod tests;

use crate::cell::UnsafeCell;
use crate::hint;
use crate::ops::{Deref, DerefMut};
use crate::sync::atomic::{Atomic, AtomicBool, Ordering};

#[derive(Default)]
pub struct SpinMutex<T> {
    value: UnsafeCell<T>,
    lock: Atomic<bool>,
}

unsafe impl<T: Send> Send for SpinMutex<T> {}
unsafe impl<T: Send> Sync for SpinMutex<T> {}

pub struct SpinMutexGuard<'a, T: 'a> {
    mutex: &'a SpinMutex<T>,
}

impl<'a, T> !Send for SpinMutexGuard<'a, T> {}
unsafe impl<'a, T: Sync> Sync for SpinMutexGuard<'a, T> {}

impl<T> SpinMutex<T> {
    pub const fn new(value: T) -> Self {
        SpinMutex { value: UnsafeCell::new(value), lock: AtomicBool::new(false) }
    }

    #[inline(always)]
    pub fn lock(&self) -> SpinMutexGuard<'_, T> {
        loop {
            match self.try_lock() {
                None => {
                    while self.lock.load(Ordering::Relaxed) {
                        hint::spin_loop()
                    }
                }
                Some(guard) => return guard,
            }
        }
    }

    #[inline(always)]
    pub fn try_lock(&self) -> Option<SpinMutexGuard<'_, T>> {
        if self.lock.compare_exchange(false, true, Ordering::Acquire, Ordering::Acquire).is_ok() {
            Some(SpinMutexGuard { mutex: self })
        } else {
            None
        }
    }
}

/// Lock the Mutex or return false.
pub macro try_lock_or_false($e:expr) {
    if let Some(v) = $e.try_lock() { v } else { return false }
}

impl<'a, T> Deref for SpinMutexGuard<'a, T> {
    type Target = T;

    fn deref(&self) -> &T {
        unsafe { &*self.mutex.value.get() }
    }
}

impl<'a, T> DerefMut for SpinMutexGuard<'a, T> {
    fn deref_mut(&mut self) -> &mut T {
        unsafe { &mut *self.mutex.value.get() }
    }
}

impl<'a, T> Drop for SpinMutexGuard<'a, T> {
    fn drop(&mut self) {
        self.mutex.lock.store(false, Ordering::Release)
    }
}