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use crate::cell::UnsafeCell;
use crate::sys::locks::{MovableCondvar, Mutex};
use crate::sys_common::lazy_box::{LazyBox, LazyInit};
pub struct RwLock {
lock: Mutex,
cond: MovableCondvar,
state: UnsafeCell<State>,
}
pub type MovableRwLock = RwLock;
enum State {
Unlocked,
Reading(usize),
Writing,
}
unsafe impl Send for RwLock {}
unsafe impl Sync for RwLock {}
// This rwlock implementation is a relatively simple implementation which has a
// condition variable for readers/writers as well as a mutex protecting the
// internal state of the lock. A current downside of the implementation is that
// unlocking the lock will notify *all* waiters rather than just readers or just
// writers. This can cause lots of "thundering stampede" problems. While
// hopefully correct this implementation is very likely to want to be changed in
// the future.
impl RwLock {
pub const fn new() -> RwLock {
RwLock {
lock: Mutex::new(),
cond: MovableCondvar::new(),
state: UnsafeCell::new(State::Unlocked),
}
}
#[inline]
pub unsafe fn read(&self) {
self.lock.lock();
while !(*self.state.get()).inc_readers() {
self.cond.wait(&self.lock);
}
self.lock.unlock();
}
#[inline]
pub unsafe fn try_read(&self) -> bool {
self.lock.lock();
let ok = (*self.state.get()).inc_readers();
self.lock.unlock();
return ok;
}
#[inline]
pub unsafe fn write(&self) {
self.lock.lock();
while !(*self.state.get()).inc_writers() {
self.cond.wait(&self.lock);
}
self.lock.unlock();
}
#[inline]
pub unsafe fn try_write(&self) -> bool {
self.lock.lock();
let ok = (*self.state.get()).inc_writers();
self.lock.unlock();
return ok;
}
#[inline]
pub unsafe fn read_unlock(&self) {
self.lock.lock();
let notify = (*self.state.get()).dec_readers();
self.lock.unlock();
if notify {
// FIXME: should only wake up one of these some of the time
self.cond.notify_all();
}
}
#[inline]
pub unsafe fn write_unlock(&self) {
self.lock.lock();
(*self.state.get()).dec_writers();
self.lock.unlock();
// FIXME: should only wake up one of these some of the time
self.cond.notify_all();
}
}
impl State {
fn inc_readers(&mut self) -> bool {
match *self {
State::Unlocked => {
*self = State::Reading(1);
true
}
State::Reading(ref mut cnt) => {
*cnt += 1;
true
}
State::Writing => false,
}
}
fn inc_writers(&mut self) -> bool {
match *self {
State::Unlocked => {
*self = State::Writing;
true
}
State::Reading(_) | State::Writing => false,
}
}
fn dec_readers(&mut self) -> bool {
let zero = match *self {
State::Reading(ref mut cnt) => {
*cnt -= 1;
*cnt == 0
}
State::Unlocked | State::Writing => invalid(),
};
if zero {
*self = State::Unlocked;
}
zero
}
fn dec_writers(&mut self) {
match *self {
State::Writing => {}
State::Unlocked | State::Reading(_) => invalid(),
}
*self = State::Unlocked;
}
}
fn invalid() -> ! {
panic!("inconsistent rwlock");
}
|
