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use crate::os::xous::ffi::do_yield;
use crate::sync::atomic::{AtomicIsize, Ordering::SeqCst};
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,
}
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(0) }
}
#[inline]
pub unsafe fn read(&self) {
while !unsafe { self.try_read() } {
do_yield();
}
}
#[inline]
pub unsafe fn try_read(&self) -> bool {
// Non-atomically determine the current value.
let current = self.mode.load(SeqCst);
// If it's currently locked for writing, then we cannot read.
if current < 0 {
return false;
}
// Attempt to lock. If the `current` value has changed, then this
// operation will fail and we will not obtain the lock even if we
// could potentially keep it.
let new = current + 1;
self.mode.compare_exchange(current, new, SeqCst, SeqCst).is_ok()
}
#[inline]
pub unsafe fn write(&self) {
while !unsafe { self.try_write() } {
do_yield();
}
}
#[inline]
pub unsafe fn try_write(&self) -> bool {
self.mode.compare_exchange(0, -1, SeqCst, SeqCst).is_ok()
}
#[inline]
pub unsafe fn read_unlock(&self) {
self.mode.fetch_sub(1, SeqCst);
}
#[inline]
pub unsafe fn write_unlock(&self) {
assert_eq!(self.mode.compare_exchange(-1, 0, SeqCst, SeqCst), Ok(-1));
}
}
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