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// Copyright 2016 Amanieu d'Antras
//
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
// http://opensource.org/licenses/MIT>, at your option. This file may not be
// copied, modified, or distributed except according to those terms.
use crate::{deadlock, util};
use core::{
sync::atomic::{AtomicU8, Ordering},
time::Duration,
};
use instant::Instant;
use lock_api::RawMutex as RawMutex_;
use parking_lot_core::{self, ParkResult, SpinWait, UnparkResult, UnparkToken, DEFAULT_PARK_TOKEN};
// UnparkToken used to indicate that that the target thread should attempt to
// lock the mutex again as soon as it is unparked.
pub(crate) const TOKEN_NORMAL: UnparkToken = UnparkToken(0);
// UnparkToken used to indicate that the mutex is being handed off to the target
// thread directly without unlocking it.
pub(crate) const TOKEN_HANDOFF: UnparkToken = UnparkToken(1);
/// This bit is set in the `state` of a `RawMutex` when that mutex is locked by some thread.
const LOCKED_BIT: u8 = 0b01;
/// This bit is set in the `state` of a `RawMutex` just before parking a thread. A thread is being
/// parked if it wants to lock the mutex, but it is currently being held by some other thread.
const PARKED_BIT: u8 = 0b10;
/// Raw mutex type backed by the parking lot.
pub struct RawMutex {
/// This atomic integer holds the current state of the mutex instance. Only the two lowest bits
/// are used. See `LOCKED_BIT` and `PARKED_BIT` for the bitmask for these bits.
///
/// # State table:
///
/// PARKED_BIT | LOCKED_BIT | Description
/// 0 | 0 | The mutex is not locked, nor is anyone waiting for it.
/// -----------+------------+------------------------------------------------------------------
/// 0 | 1 | The mutex is locked by exactly one thread. No other thread is
/// | | waiting for it.
/// -----------+------------+------------------------------------------------------------------
/// 1 | 0 | The mutex is not locked. One or more thread is parked or about to
/// | | park. At least one of the parked threads are just about to be
/// | | unparked, or a thread heading for parking might abort the park.
/// -----------+------------+------------------------------------------------------------------
/// 1 | 1 | The mutex is locked by exactly one thread. One or more thread is
/// | | parked or about to park, waiting for the lock to become available.
/// | | In this state, PARKED_BIT is only ever cleared when a bucket lock
/// | | is held (i.e. in a parking_lot_core callback). This ensures that
/// | | we never end up in a situation where there are parked threads but
/// | | PARKED_BIT is not set (which would result in those threads
/// | | potentially never getting woken up).
state: AtomicU8,
}
unsafe impl lock_api::RawMutex for RawMutex {
const INIT: RawMutex = RawMutex {
state: AtomicU8::new(0),
};
type GuardMarker = crate::GuardMarker;
#[inline]
fn lock(&self) {
if self
.state
.compare_exchange_weak(0, LOCKED_BIT, Ordering::Acquire, Ordering::Relaxed)
.is_err()
{
self.lock_slow(None);
}
unsafe { deadlock::acquire_resource(self as *const _ as usize) };
}
#[inline]
fn try_lock(&self) -> bool {
let mut state = self.state.load(Ordering::Relaxed);
loop {
if state & LOCKED_BIT != 0 {
return false;
}
match self.state.compare_exchange_weak(
state,
state | LOCKED_BIT,
Ordering::Acquire,
Ordering::Relaxed,
) {
Ok(_) => {
unsafe { deadlock::acquire_resource(self as *const _ as usize) };
return true;
}
Err(x) => state = x,
}
}
}
#[inline]
unsafe fn unlock(&self) {
deadlock::release_resource(self as *const _ as usize);
if self
.state
.compare_exchange(LOCKED_BIT, 0, Ordering::Release, Ordering::Relaxed)
.is_ok()
{
return;
}
self.unlock_slow(false);
}
#[inline]
fn is_locked(&self) -> bool {
let state = self.state.load(Ordering::Relaxed);
state & LOCKED_BIT != 0
}
}
unsafe impl lock_api::RawMutexFair for RawMutex {
#[inline]
unsafe fn unlock_fair(&self) {
deadlock::release_resource(self as *const _ as usize);
if self
.state
.compare_exchange(LOCKED_BIT, 0, Ordering::Release, Ordering::Relaxed)
.is_ok()
{
return;
}
self.unlock_slow(true);
}
#[inline]
unsafe fn bump(&self) {
if self.state.load(Ordering::Relaxed) & PARKED_BIT != 0 {
self.bump_slow();
}
}
}
unsafe impl lock_api::RawMutexTimed for RawMutex {
type Duration = Duration;
type Instant = Instant;
#[inline]
fn try_lock_until(&self, timeout: Instant) -> bool {
let result = if self
.state
.compare_exchange_weak(0, LOCKED_BIT, Ordering::Acquire, Ordering::Relaxed)
.is_ok()
{
true
} else {
self.lock_slow(Some(timeout))
};
if result {
unsafe { deadlock::acquire_resource(self as *const _ as usize) };
}
result
}
#[inline]
fn try_lock_for(&self, timeout: Duration) -> bool {
let result = if self
.state
.compare_exchange_weak(0, LOCKED_BIT, Ordering::Acquire, Ordering::Relaxed)
.is_ok()
{
true
} else {
self.lock_slow(util::to_deadline(timeout))
};
if result {
unsafe { deadlock::acquire_resource(self as *const _ as usize) };
}
result
}
}
impl RawMutex {
// Used by Condvar when requeuing threads to us, must be called while
// holding the queue lock.
#[inline]
pub(crate) fn mark_parked_if_locked(&self) -> bool {
let mut state = self.state.load(Ordering::Relaxed);
loop {
if state & LOCKED_BIT == 0 {
return false;
}
match self.state.compare_exchange_weak(
state,
state | PARKED_BIT,
Ordering::Relaxed,
Ordering::Relaxed,
) {
Ok(_) => return true,
Err(x) => state = x,
}
}
}
// Used by Condvar when requeuing threads to us, must be called while
// holding the queue lock.
#[inline]
pub(crate) fn mark_parked(&self) {
self.state.fetch_or(PARKED_BIT, Ordering::Relaxed);
}
#[cold]
fn lock_slow(&self, timeout: Option<Instant>) -> bool {
let mut spinwait = SpinWait::new();
let mut state = self.state.load(Ordering::Relaxed);
loop {
// Grab the lock if it isn't locked, even if there is a queue on it
if state & LOCKED_BIT == 0 {
match self.state.compare_exchange_weak(
state,
state | LOCKED_BIT,
Ordering::Acquire,
Ordering::Relaxed,
) {
Ok(_) => return true,
Err(x) => state = x,
}
continue;
}
// If there is no queue, try spinning a few times
if state & PARKED_BIT == 0 && spinwait.spin() {
state = self.state.load(Ordering::Relaxed);
continue;
}
// Set the parked bit
if state & PARKED_BIT == 0 {
if let Err(x) = self.state.compare_exchange_weak(
state,
state | PARKED_BIT,
Ordering::Relaxed,
Ordering::Relaxed,
) {
state = x;
continue;
}
}
// Park our thread until we are woken up by an unlock
let addr = self as *const _ as usize;
let validate = || self.state.load(Ordering::Relaxed) == LOCKED_BIT | PARKED_BIT;
let before_sleep = || {};
let timed_out = |_, was_last_thread| {
// Clear the parked bit if we were the last parked thread
if was_last_thread {
self.state.fetch_and(!PARKED_BIT, Ordering::Relaxed);
}
};
// SAFETY:
// * `addr` is an address we control.
// * `validate`/`timed_out` does not panic or call into any function of `parking_lot`.
// * `before_sleep` does not call `park`, nor does it panic.
match unsafe {
parking_lot_core::park(
addr,
validate,
before_sleep,
timed_out,
DEFAULT_PARK_TOKEN,
timeout,
)
} {
// The thread that unparked us passed the lock on to us
// directly without unlocking it.
ParkResult::Unparked(TOKEN_HANDOFF) => return true,
// We were unparked normally, try acquiring the lock again
ParkResult::Unparked(_) => (),
// The validation function failed, try locking again
ParkResult::Invalid => (),
// Timeout expired
ParkResult::TimedOut => return false,
}
// Loop back and try locking again
spinwait.reset();
state = self.state.load(Ordering::Relaxed);
}
}
#[cold]
fn unlock_slow(&self, force_fair: bool) {
// Unpark one thread and leave the parked bit set if there might
// still be parked threads on this address.
let addr = self as *const _ as usize;
let callback = |result: UnparkResult| {
// If we are using a fair unlock then we should keep the
// mutex locked and hand it off to the unparked thread.
if result.unparked_threads != 0 && (force_fair || result.be_fair) {
// Clear the parked bit if there are no more parked
// threads.
if !result.have_more_threads {
self.state.store(LOCKED_BIT, Ordering::Relaxed);
}
return TOKEN_HANDOFF;
}
// Clear the locked bit, and the parked bit as well if there
// are no more parked threads.
if result.have_more_threads {
self.state.store(PARKED_BIT, Ordering::Release);
} else {
self.state.store(0, Ordering::Release);
}
TOKEN_NORMAL
};
// SAFETY:
// * `addr` is an address we control.
// * `callback` does not panic or call into any function of `parking_lot`.
unsafe {
parking_lot_core::unpark_one(addr, callback);
}
}
#[cold]
fn bump_slow(&self) {
unsafe { deadlock::release_resource(self as *const _ as usize) };
self.unlock_slow(true);
self.lock();
}
}