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    use crate::cell::UnsafeCell;
    use crate::sync::atomic::{AtomicUsize, Ordering};

    pub struct RWLock {
        inner: UnsafeCell<libc::pthread_rwlock_t>,
        write_locked: UnsafeCell<bool>, // guarded by the `inner` RwLock
        num_readers: AtomicUsize,
    }

    unsafe impl Send for RWLock {}
    unsafe impl Sync for RWLock {}

    impl RWLock {
        pub const fn new() -> RWLock {
            RWLock {
                inner: UnsafeCell::new(libc::PTHREAD_RWLOCK_INITIALIZER),
                write_locked: UnsafeCell::new(false),
                num_readers: AtomicUsize::new(0),
            }
        }
        #[inline]
        pub unsafe fn read(&self) {
            let r = libc::pthread_rwlock_rdlock(self.inner.get());

            // According to the pthread_rwlock_rdlock spec, this function **may**
            // fail with EDEADLK if a deadlock is detected. On the other hand
            // pthread mutexes will *never* return EDEADLK if they are initialized
            // as the "fast" kind (which ours always are). As a result, a deadlock
            // situation may actually return from the call to pthread_rwlock_rdlock
            // instead of blocking forever (as mutexes and Windows rwlocks do). Note
            // that not all unix implementations, however, will return EDEADLK for
            // their rwlocks.
            //
            // We roughly maintain the deadlocking behavior by panicking to ensure
            // that this lock acquisition does not succeed.
            //
            // We also check whether this lock is already write locked. This
            // is only possible if it was write locked by the current thread and
            // the implementation allows recursive locking. The POSIX standard
            // doesn't require recursively locking a rwlock to deadlock, but we can't
            // allow that because it could lead to aliasing issues.
            if r == libc::EAGAIN {
                panic!("rwlock maximum reader count exceeded");
            } else if r == libc::EDEADLK || (r == 0 && *self.write_locked.get()) {
                if r == 0 {
                    self.raw_unlock();
                }
                panic!("rwlock read lock would result in deadlock");
            } else {
                assert_eq!(r, 0);
                self.num_readers.fetch_add(1, Ordering::Relaxed);
            }
        }
        #[inline]
        pub unsafe fn try_read(&self) -> bool {
            let r = libc::pthread_rwlock_tryrdlock(self.inner.get());
            if r == 0 {
                if *self.write_locked.get() {
                    self.raw_unlock();
                    false
                } else {
                    self.num_readers.fetch_add(1, Ordering::Relaxed);
                    true
                }
            } else {
                false
            }
        }
        #[inline]
        pub unsafe fn write(&self) {
            let r = libc::pthread_rwlock_wrlock(self.inner.get());
            // See comments above for why we check for EDEADLK and write_locked. We
            // also need to check that num_readers is 0.
            if r == libc::EDEADLK || *self.write_locked.get() ||
               self.num_readers.load(Ordering::Relaxed) != 0 {
                if r == 0 {
                    self.raw_unlock();
                }
                panic!("rwlock write lock would result in deadlock");
            } else {
                debug_assert_eq!(r, 0);
            }
            *self.write_locked.get() = true;
        }
        #[inline]
        pub unsafe fn try_write(&self) -> bool {
            let r = libc::pthread_rwlock_trywrlock(self.inner.get());
            if r == 0 {
                if *self.write_locked.get() || self.num_readers.load(Ordering::Relaxed) != 0 {
                    self.raw_unlock();
                    false
                } else {
                    *self.write_locked.get() = true;
                    true
                }
            } else {
                false
            }
        }
        #[inline]
        unsafe fn raw_unlock(&self) {
            let r = libc::pthread_rwlock_unlock(self.inner.get());
            debug_assert_eq!(r, 0);
        }
        #[inline]
        pub unsafe fn read_unlock(&self) {
            debug_assert!(!*self.write_locked.get());
            self.num_readers.fetch_sub(1, Ordering::Relaxed);
            self.raw_unlock();
        }
        #[inline]
        pub unsafe fn write_unlock(&self) {
            debug_assert_eq!(self.num_readers.load(Ordering::Relaxed), 0);
            debug_assert!(*self.write_locked.get());
            *self.write_locked.get() = false;
            self.raw_unlock();
        }
        #[inline]
        pub unsafe fn destroy(&self) {
            let r = libc::pthread_rwlock_destroy(self.inner.get());
            // On DragonFly pthread_rwlock_destroy() returns EINVAL if called on a
            // rwlock that was just initialized with
            // libc::PTHREAD_RWLOCK_INITIALIZER. Once it is used (locked/unlocked)
            // or pthread_rwlock_init() is called, this behaviour no longer occurs.
            if cfg!(target_os = "dragonfly") {
                debug_assert!(r == 0 || r == libc::EINVAL);
            } else {
                debug_assert_eq!(r, 0);
            }
        }
    }