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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
//! An implementation of re-entrant mutexes.
//!
//! Re-entrant mutexes are like mutexes, but where it is expected
//! that a single thread may own a lock more than once.
//! It provides the same interface as https://github.com/rust-lang/rust/blob/master/src/libstd/sys/common/remutex.rs
//! so if those types are ever exported, we should be able to replace this implemtation.
use core::nonzero::NonZero;
use std::cell::{Cell, UnsafeCell};
use std::mem;
use std::ops::Deref;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::sync::{LockResult, Mutex, MutexGuard, PoisonError, TryLockError, TryLockResult};
/// A type for thread ids.
// TODO: can we use the thread-id crate for this?
#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Debug)]
pub struct ThreadId(NonZero<usize>);
lazy_static!{ static ref THREAD_COUNT: AtomicUsize = AtomicUsize::new(1); }
impl ThreadId {
#[allow(unsafe_code)]
fn new() -> ThreadId {
let number = THREAD_COUNT.fetch_add(1, Ordering::SeqCst);
ThreadId(unsafe { NonZero::new(number) })
}
pub fn current() -> ThreadId {
THREAD_ID.with(|tls| tls.clone())
}
}
thread_local!{ static THREAD_ID: ThreadId = ThreadId::new() }
/// A type for atomic storage of thread ids.
#[derive(Debug)]
pub struct AtomicOptThreadId(AtomicUsize);
impl AtomicOptThreadId {
pub fn new() -> AtomicOptThreadId {
AtomicOptThreadId(AtomicUsize::new(0))
}
pub fn store(&self, value: Option<ThreadId>, ordering: Ordering) {
let number = value.map(|id| *id.0).unwrap_or(0);
self.0.store(number, ordering);
}
#[allow(unsafe_code)]
pub fn load(&self, ordering: Ordering) -> Option<ThreadId> {
let number = self.0.load(ordering);
if number == 0 { None } else { Some(ThreadId(unsafe { NonZero::new(number) })) }
}
#[allow(unsafe_code)]
pub fn swap(&self, value: Option<ThreadId>, ordering: Ordering) -> Option<ThreadId> {
let number = value.map(|id| *id.0).unwrap_or(0);
let number = self.0.swap(number, ordering);
if number == 0 { None } else { Some(ThreadId(unsafe { NonZero::new(number) })) }
}
}
/// A type for hand-over-hand mutexes.
///
/// These support `lock` and `unlock` functions. `lock` blocks waiting to become the
/// mutex owner. `unlock` can only be called by the lock owner, and panics otherwise.
/// They have the same happens-before and poisoning semantics as `Mutex`.
// TODO: Can we use `raw_lock` and `raw_unlock` from `parking_lot`'s `Mutex` for this?
pub struct HandOverHandMutex {
mutex: Mutex<()>,
owner: AtomicOptThreadId,
guard: UnsafeCell<Option<MutexGuard<'static, ()>>>,
}
impl HandOverHandMutex {
pub fn new() -> HandOverHandMutex {
HandOverHandMutex {
mutex: Mutex::new(()),
owner: AtomicOptThreadId::new(),
guard: UnsafeCell::new(None),
}
}
#[allow(unsafe_code)]
pub fn lock(&self) -> LockResult<()> {
let (guard, result) = match self.mutex.lock() {
Ok(guard) => (guard, Ok(())),
Err(err) => (err.into_inner(), Err(PoisonError::new(()))),
};
unsafe { *self.guard.get().as_mut().unwrap() = mem::transmute(guard) };
self.owner.store(Some(ThreadId::current()), Ordering::Relaxed);
result
}
#[allow(unsafe_code)]
pub fn try_lock(&self) -> TryLockResult<()> {
let (guard, result) = match self.mutex.try_lock() {
Ok(guard) => (guard, Ok(())),
Err(TryLockError::WouldBlock) => return Err(TryLockError::WouldBlock),
Err(TryLockError::Poisoned(err)) => (err.into_inner(), Err(TryLockError::Poisoned(PoisonError::new(())))),
};
unsafe { *self.guard.get().as_mut().unwrap() = mem::transmute(guard) };
self.owner.store(Some(ThreadId::current()), Ordering::Relaxed);
result
}
#[allow(unsafe_code)]
pub fn unlock(&self) {
assert_eq!(Some(ThreadId::current()), self.owner.load(Ordering::Relaxed));
self.owner.store(None, Ordering::Relaxed);
unsafe { *self.guard.get().as_mut().unwrap() = None; }
}
pub fn owner(&self) -> Option<ThreadId> {
self.owner.load(Ordering::Relaxed)
}
}
#[allow(unsafe_code)]
unsafe impl Send for HandOverHandMutex {}
/// A type for re-entrant mutexes.
///
/// It provides the same interface as https://github.com/rust-lang/rust/blob/master/src/libstd/sys/common/remutex.rs
pub struct ReentrantMutex<T> {
mutex: HandOverHandMutex,
count: Cell<usize>,
data: T,
}
#[allow(unsafe_code)]
unsafe impl<T> Sync for ReentrantMutex<T> where T: Send {}
impl<T> ReentrantMutex<T> {
pub fn new(data: T) -> ReentrantMutex<T> {
debug!("{:?} Creating new lock.", ThreadId::current());
ReentrantMutex {
mutex: HandOverHandMutex::new(),
count: Cell::new(0),
data: data,
}
}
pub fn lock(&self) -> LockResult<ReentrantMutexGuard<T>> {
debug!("{:?} Locking.", ThreadId::current());
if self.mutex.owner() != Some(ThreadId::current()) {
debug!("{:?} Becoming owner.", ThreadId::current());
if let Err(_) = self.mutex.lock() {
debug!("{:?} Poison!", ThreadId::current());
return Err(PoisonError::new(self.mk_guard()));
}
debug!("{:?} Became owner.", ThreadId::current());
}
Ok(self.mk_guard())
}
pub fn try_lock(&self) -> TryLockResult<ReentrantMutexGuard<T>> {
debug!("{:?} Try locking.", ThreadId::current());
if self.mutex.owner() != Some(ThreadId::current()) {
debug!("{:?} Becoming owner?", ThreadId::current());
if let Err(err) = self.mutex.try_lock() {
match err {
TryLockError::WouldBlock => {
debug!("{:?} Would block.", ThreadId::current());
return Err(TryLockError::WouldBlock)
},
TryLockError::Poisoned(_) => {
debug!("{:?} Poison!", ThreadId::current());
return Err(TryLockError::Poisoned(PoisonError::new(self.mk_guard())));
},
}
}
debug!("{:?} Became owner.", ThreadId::current());
}
Ok(self.mk_guard())
}
fn unlock(&self) {
debug!("{:?} Unlocking.", ThreadId::current());
let count = self.count.get().checked_sub(1).expect("Underflowed lock count.");
debug!("{:?} Decrementing count to {}.", ThreadId::current(), count);
self.count.set(count);
if count == 0 {
debug!("{:?} Releasing mutex.", ThreadId::current());
self.mutex.unlock();
}
}
fn mk_guard(&self) -> ReentrantMutexGuard<T> {
let count = self.count.get().checked_add(1).expect("Overflowed lock count.");
debug!("{:?} Incrementing count to {}.", ThreadId::current(), count);
self.count.set(count);
ReentrantMutexGuard { mutex: self }
}
}
#[must_use]
pub struct ReentrantMutexGuard<'a, T> where T: 'static {
mutex: &'a ReentrantMutex<T>,
}
impl<'a, T> Drop for ReentrantMutexGuard<'a, T> {
#[allow(unsafe_code)]
fn drop(&mut self) {
self.mutex.unlock()
}
}
impl<'a, T> Deref for ReentrantMutexGuard<'a, T> {
type Target = T;
fn deref(&self) -> &T {
&self.mutex.data
}
}
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