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use crate::sync::atomic::{AtomicIsize, Ordering::Acquire};
use crate::thread::yield_now;
pub struct RwLock {
/// The "mode" value indicates how many threads are waiting on this
/// Mutex. Possible values are:
/// -1: The lock is locked for writing
/// 0: The lock is unlocked
/// >=1: The lock is locked for reading
///
/// This currently spins waiting for the lock to be freed. An
/// optimization would be to involve the ticktimer server to
/// coordinate unlocks.
mode: AtomicIsize,
}
const RWLOCK_WRITING: isize = -1;
const RWLOCK_FREE: isize = 0;
unsafe impl Send for RwLock {}
unsafe impl Sync for RwLock {}
impl RwLock {
#[inline]
#[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
pub const fn new() -> RwLock {
RwLock { mode: AtomicIsize::new(RWLOCK_FREE) }
}
#[inline]
pub unsafe fn read(&self) {
while !unsafe { self.try_read() } {
yield_now();
}
}
#[inline]
pub unsafe fn try_read(&self) -> bool {
self.mode
.fetch_update(
Acquire,
Acquire,
|v| if v == RWLOCK_WRITING { None } else { Some(v + 1) },
)
.is_ok()
}
#[inline]
pub unsafe fn write(&self) {
while !unsafe { self.try_write() } {
yield_now();
}
}
#[inline]
pub unsafe fn try_write(&self) -> bool {
self.mode.compare_exchange(RWLOCK_FREE, RWLOCK_WRITING, Acquire, Acquire).is_ok()
}
#[inline]
pub unsafe fn read_unlock(&self) {
let previous = self.mode.fetch_sub(1, Acquire);
assert!(previous != RWLOCK_FREE);
assert!(previous != RWLOCK_WRITING);
}
#[inline]
pub unsafe fn write_unlock(&self) {
assert_eq!(
self.mode.compare_exchange(RWLOCK_WRITING, RWLOCK_FREE, Acquire, Acquire),
Ok(RWLOCK_WRITING)
);
}
}
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