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use core::sync::atomic::{AtomicBool, Ordering, spin_loop_hint as cpu_relax};
use core::cell::UnsafeCell;
use core::marker::Sync;
use core::ops::{Drop, Deref, DerefMut};
use core::fmt;
use core::option::Option::{self, None, Some};
use core::default::Default;
/// This type provides MUTual EXclusion based on spinning.
///
/// # Description
///
/// The behaviour of these lock is similar to their namesakes in `std::sync`. they
/// differ on the following:
///
/// - The lock will not be poisoned in case of failure;
///
/// # Simple examples
///
/// ```
/// use spin;
/// let spin_mutex = spin::Mutex::new(0);
///
/// // Modify the data
/// {
/// let mut data = spin_mutex.lock();
/// *data = 2;
/// }
///
/// // Read the data
/// let answer =
/// {
/// let data = spin_mutex.lock();
/// *data
/// };
///
/// assert_eq!(answer, 2);
/// ```
///
/// # Thread-safety example
///
/// ```
/// use spin;
/// use std::sync::{Arc, Barrier};
///
/// let numthreads = 1000;
/// let spin_mutex = Arc::new(spin::Mutex::new(0));
///
/// // We use a barrier to ensure the readout happens after all writing
/// let barrier = Arc::new(Barrier::new(numthreads + 1));
///
/// for _ in (0..numthreads)
/// {
/// let my_barrier = barrier.clone();
/// let my_lock = spin_mutex.clone();
/// std::thread::spawn(move||
/// {
/// let mut guard = my_lock.lock();
/// *guard += 1;
///
/// // Release the lock to prevent a deadlock
/// drop(guard);
/// my_barrier.wait();
/// });
/// }
///
/// barrier.wait();
///
/// let answer = { *spin_mutex.lock() };
/// assert_eq!(answer, numthreads);
/// ```
pub struct Mutex<T: ?Sized>
{
lock: AtomicBool,
data: UnsafeCell<T>,
}
/// A guard to which the protected data can be accessed
///
/// When the guard falls out of scope it will release the lock.
#[derive(Debug)]
pub struct MutexGuard<'a, T: ?Sized + 'a>
{
lock: &'a AtomicBool,
data: &'a mut T,
}
// Same unsafe impls as `std::sync::Mutex`
unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}
unsafe impl<T: ?Sized + Send> Send for Mutex<T> {}
impl<T> Mutex<T>
{
/// Creates a new spinlock wrapping the supplied data.
///
/// May be used statically:
///
/// ```
/// use spin;
///
/// static MUTEX: spin::Mutex<()> = spin::Mutex::new(());
///
/// fn demo() {
/// let lock = MUTEX.lock();
/// // do something with lock
/// drop(lock);
/// }
/// ```
pub const fn new(user_data: T) -> Mutex<T>
{
Mutex
{
lock: AtomicBool::new(false),
data: UnsafeCell::new(user_data),
}
}
/// Consumes this mutex, returning the underlying data.
pub fn into_inner(self) -> T {
// We know statically that there are no outstanding references to
// `self` so there's no need to lock.
let Mutex { data, .. } = self;
data.into_inner()
}
}
impl<T: ?Sized> Mutex<T>
{
fn obtain_lock(&self)
{
while self.lock.compare_and_swap(false, true, Ordering::Acquire) != false
{
// Wait until the lock looks unlocked before retrying
while self.lock.load(Ordering::Relaxed)
{
cpu_relax();
}
}
}
/// Locks the spinlock and returns a guard.
///
/// The returned value may be dereferenced for data access
/// and the lock will be dropped when the guard falls out of scope.
///
/// ```
/// let mylock = spin::Mutex::new(0);
/// {
/// let mut data = mylock.lock();
/// // The lock is now locked and the data can be accessed
/// *data += 1;
/// // The lock is implicitly dropped
/// }
///
/// ```
pub fn lock(&self) -> MutexGuard<T>
{
self.obtain_lock();
MutexGuard
{
lock: &self.lock,
data: unsafe { &mut *self.data.get() },
}
}
/// Force unlock the spinlock.
///
/// This is *extremely* unsafe if the lock is not held by the current
/// thread. However, this can be useful in some instances for exposing the
/// lock to FFI that doesn't know how to deal with RAII.
///
/// If the lock isn't held, this is a no-op.
pub unsafe fn force_unlock(&self) {
self.lock.store(false, Ordering::Release);
}
/// Tries to lock the mutex. If it is already locked, it will return None. Otherwise it returns
/// a guard within Some.
pub fn try_lock(&self) -> Option<MutexGuard<T>>
{
if self.lock.compare_and_swap(false, true, Ordering::Acquire) == false
{
Some(
MutexGuard {
lock: &self.lock,
data: unsafe { &mut *self.data.get() },
}
)
}
else
{
None
}
}
}
impl<T: ?Sized + fmt::Debug> fmt::Debug for Mutex<T>
{
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result
{
match self.try_lock()
{
Some(guard) => write!(f, "Mutex {{ data: ")
.and_then(|()| (&*guard).fmt(f))
.and_then(|()| write!(f, "}}")),
None => write!(f, "Mutex {{ <locked> }}"),
}
}
}
impl<T: ?Sized + Default> Default for Mutex<T> {
fn default() -> Mutex<T> {
Mutex::new(Default::default())
}
}
impl<'a, T: ?Sized> Deref for MutexGuard<'a, T>
{
type Target = T;
fn deref<'b>(&'b self) -> &'b T { &*self.data }
}
impl<'a, T: ?Sized> DerefMut for MutexGuard<'a, T>
{
fn deref_mut<'b>(&'b mut self) -> &'b mut T { &mut *self.data }
}
impl<'a, T: ?Sized> Drop for MutexGuard<'a, T>
{
/// The dropping of the MutexGuard will release the lock it was created from.
fn drop(&mut self)
{
self.lock.store(false, Ordering::Release);
}
}
#[cfg(test)]
mod tests {
use std::prelude::v1::*;
use std::sync::mpsc::channel;
use std::sync::Arc;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::thread;
use super::*;
#[derive(Eq, PartialEq, Debug)]
struct NonCopy(i32);
#[test]
fn smoke() {
let m = Mutex::new(());
drop(m.lock());
drop(m.lock());
}
#[test]
fn lots_and_lots() {
static M: Mutex<()> = Mutex::new(());
static mut CNT: u32 = 0;
const J: u32 = 1000;
const K: u32 = 3;
fn inc() {
for _ in 0..J {
unsafe {
let _g = M.lock();
CNT += 1;
}
}
}
let (tx, rx) = channel();
for _ in 0..K {
let tx2 = tx.clone();
thread::spawn(move|| { inc(); tx2.send(()).unwrap(); });
let tx2 = tx.clone();
thread::spawn(move|| { inc(); tx2.send(()).unwrap(); });
}
drop(tx);
for _ in 0..2 * K {
rx.recv().unwrap();
}
assert_eq!(unsafe {CNT}, J * K * 2);
}
#[test]
fn try_lock() {
let mutex = Mutex::new(42);
// First lock succeeds
let a = mutex.try_lock();
assert_eq!(a.as_ref().map(|r| **r), Some(42));
// Additional lock failes
let b = mutex.try_lock();
assert!(b.is_none());
// After dropping lock, it succeeds again
::core::mem::drop(a);
let c = mutex.try_lock();
assert_eq!(c.as_ref().map(|r| **r), Some(42));
}
#[test]
fn test_into_inner() {
let m = Mutex::new(NonCopy(10));
assert_eq!(m.into_inner(), NonCopy(10));
}
#[test]
fn test_into_inner_drop() {
struct Foo(Arc<AtomicUsize>);
impl Drop for Foo {
fn drop(&mut self) {
self.0.fetch_add(1, Ordering::SeqCst);
}
}
let num_drops = Arc::new(AtomicUsize::new(0));
let m = Mutex::new(Foo(num_drops.clone()));
assert_eq!(num_drops.load(Ordering::SeqCst), 0);
{
let _inner = m.into_inner();
assert_eq!(num_drops.load(Ordering::SeqCst), 0);
}
assert_eq!(num_drops.load(Ordering::SeqCst), 1);
}
#[test]
fn test_mutex_arc_nested() {
// Tests nested mutexes and access
// to underlying data.
let arc = Arc::new(Mutex::new(1));
let arc2 = Arc::new(Mutex::new(arc));
let (tx, rx) = channel();
let _t = thread::spawn(move|| {
let lock = arc2.lock();
let lock2 = lock.lock();
assert_eq!(*lock2, 1);
tx.send(()).unwrap();
});
rx.recv().unwrap();
}
#[test]
fn test_mutex_arc_access_in_unwind() {
let arc = Arc::new(Mutex::new(1));
let arc2 = arc.clone();
let _ = thread::spawn(move|| -> () {
struct Unwinder {
i: Arc<Mutex<i32>>,
}
impl Drop for Unwinder {
fn drop(&mut self) {
*self.i.lock() += 1;
}
}
let _u = Unwinder { i: arc2 };
panic!();
}).join();
let lock = arc.lock();
assert_eq!(*lock, 2);
}
#[test]
fn test_mutex_unsized() {
let mutex: &Mutex<[i32]> = &Mutex::new([1, 2, 3]);
{
let b = &mut *mutex.lock();
b[0] = 4;
b[2] = 5;
}
let comp: &[i32] = &[4, 2, 5];
assert_eq!(&*mutex.lock(), comp);
}
#[test]
fn test_mutex_force_lock() {
let lock = Mutex::new(());
::std::mem::forget(lock.lock());
unsafe {
lock.force_unlock();
}
assert!(lock.try_lock().is_some());
}
}