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use core::{
fmt,
sync::atomic::{AtomicUsize, Ordering},
};
use super::{Lock, SendMarker};
#[cfg_attr(docsrs, doc(cfg(feature = "atomic")))]
pub struct AtomicLock {
count: AtomicUsize,
}
const EXCLUSIVE_FLAG: usize = !(usize::max_value() >> 1);
impl fmt::Debug for AtomicLock {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let count = self.count.load(Ordering::Relaxed);
if (count & EXCLUSIVE_FLAG) != 0 {
write!(f, "AtomicLock {{ <locked exclusively> }}",)
} else {
write!(f, "AtomicLock {{ num_shared_locks: {} }}", count)
}
}
}
unsafe impl Lock for AtomicLock {
type LockMarker = SendMarker;
type UnlockMarker = SendMarker;
#[inline]
fn new() -> Self {
Self {
count: AtomicUsize::new(0),
}
}
#[inline]
unsafe fn lock_shared(&self) {
if !self.try_lock_shared() {
borrow_fail();
}
}
#[inline]
unsafe fn try_lock_shared(&self) -> bool {
let old_count = self.count.fetch_add(1, Ordering::Acquire);
if old_count < EXCLUSIVE_FLAG / 2 {
return true;
}
self.count.fetch_sub(1, Ordering::Relaxed);
false
}
#[inline]
unsafe fn unlock_shared(&self) {
let old_count = self.count.fetch_sub(1, Ordering::Release);
debug_assert!((old_count & EXCLUSIVE_FLAG) == 0);
}
#[inline]
unsafe fn lock_exclusive(&self) {
if !self.try_lock_exclusive() {
borrow_fail();
}
}
#[inline]
unsafe fn try_lock_exclusive(&self) -> bool {
self.count
.compare_exchange(0, EXCLUSIVE_FLAG, Ordering::Acquire, Ordering::Relaxed)
.is_ok()
}
#[inline]
unsafe fn unlock_exclusive(&self) {
let old_count = self.count.fetch_sub(EXCLUSIVE_FLAG, Ordering::Release);
debug_assert!((old_count & EXCLUSIVE_FLAG) != 0);
}
}
#[cold]
fn borrow_fail() -> ! {
panic!("locked")
}