use core::sync::atomic::{AtomicUsize, Ordering, ATOMIC_USIZE_INIT, spin_loop_hint as cpu_relax};
use core::cell::UnsafeCell;
use core::ops::{Deref, DerefMut};
use core::fmt;
use core::default::Default;
/// A reader-writer lock
///
/// This type of lock allows a number of readers or at most one writer at any
/// point in time. The write portion of this lock typically allows modification
/// of the underlying data (exclusive access) and the read portion of this lock
/// typically allows for read-only access (shared access).
///
/// The type parameter `T` represents the data that this lock protects. It is
/// required that `T` satisfies `Send` to be shared across tasks and `Sync` to
/// allow concurrent access through readers. The RAII guards returned from the
/// locking methods implement `Deref` (and `DerefMut` for the `write` methods)
/// to allow access to the contained of the lock.
///
/// Based on
/// https://jfdube.wordpress.com/2014/01/03/implementing-a-recursive-read-write-spinlock/
///
/// # Examples
///
/// ```
/// use spin;
///
/// let lock = spin::RwLock::new(5);
///
/// // many reader locks can be held at once
/// {
/// let r1 = lock.read();
/// let r2 = lock.read();
/// assert_eq!(*r1, 5);
/// assert_eq!(*r2, 5);
/// } // read locks are dropped at this point
///
/// // only one write lock may be held, however
/// {
/// let mut w = lock.write();
/// *w += 1;
/// assert_eq!(*w, 6);
/// } // write lock is dropped here
/// ```
pub struct RwLock<T: ?Sized>
{
lock: AtomicUsize,
data: UnsafeCell<T>,
}
/// A guard to which the protected data can be read
///
/// When the guard falls out of scope it will decrement the read count,
/// potentially releasing the lock.
pub struct RwLockReadGuard<'a, T: 'a + ?Sized>
{
lock: &'a AtomicUsize,
data: &'a T,
}
/// A guard to which the protected data can be written
///
/// When the guard falls out of scope it will release the lock.
pub struct RwLockWriteGuard<'a, T: 'a + ?Sized>
{
lock: &'a AtomicUsize,
data: &'a mut T,
}
// Same unsafe impls as `std::sync::RwLock`
unsafe impl<T: ?Sized + Send> Send for RwLock<T> {}
unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}
const USIZE_MSB: usize = ::core::isize::MIN as usize;
impl<T> RwLock<T>
{
/// Creates a new spinlock wrapping the supplied data.
///
/// May be used statically:
///
/// ```
/// #![feature(const_fn)]
/// use spin;
///
/// static RW_LOCK: spin::RwLock<()> = spin::RwLock::new(());
///
/// fn demo() {
/// let lock = RW_LOCK.read();
/// // do something with lock
/// drop(lock);
/// }
/// ```
#[inline]
#[cfg(feature = "const_fn")]
pub const fn new(user_data: T) -> RwLock<T>
{
RwLock
{
lock: ATOMIC_USIZE_INIT,
data: UnsafeCell::new(user_data),
}
}
/// Creates a new spinlock wrapping the supplied data.
///
/// If you want to use it statically, you can use the `const_fn` feature.
///
/// ```
/// use spin;
///
/// fn demo() {
/// let rw_lock = spin::RwLock::new(());
/// let lock = rw_lock.read();
/// // do something with lock
/// drop(lock);
/// }
/// ```
#[inline]
#[cfg(not(feature = "const_fn"))]
pub fn new(user_data: T) -> RwLock<T>
{
RwLock
{
lock: ATOMIC_USIZE_INIT,
data: UnsafeCell::new(user_data),
}
}
/// Consumes this `RwLock`, 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 RwLock { data, .. } = self;
data.into_inner()
}
}
impl<T: ?Sized> RwLock<T>
{
/// Locks this rwlock with shared read access, blocking the current thread
/// until it can be acquired.
///
/// The calling thread will be blocked until there are no more writers which
/// hold the lock. There may be other readers currently inside the lock when
/// this method returns. This method does not provide any guarantees with
/// respect to the ordering of whether contentious readers or writers will
/// acquire the lock first.
///
/// Returns an RAII guard which will release this thread's shared access
/// once it is dropped.
///
/// ```
/// let mylock = spin::RwLock::new(0);
/// {
/// let mut data = mylock.read();
/// // The lock is now locked and the data can be read
/// println!("{}", *data);
/// // The lock is dropped
/// }
/// ```
#[inline]
pub fn read<'a>(&'a self) -> RwLockReadGuard<'a, T>
{
// (funny do-while loop)
while {
// Old value, with write bit unset
let mut old;
// Wait for for writer to go away before doing expensive atomic ops
// (funny do-while loop)
while {
old = self.lock.load(Ordering::Relaxed);
old & USIZE_MSB != 0
} {
cpu_relax();
}
// unset write bit
old &= !USIZE_MSB;
let new = old + 1;
debug_assert!(new != (!USIZE_MSB) & (!0));
self.lock.compare_and_swap(old, new, Ordering::SeqCst) != old
} {
cpu_relax();
}
RwLockReadGuard {
lock: &self.lock,
data: unsafe { & *self.data.get() },
}
}
/// Attempt to acquire this lock with shared read access.
///
/// This function will never block and will return immediately if `read`
/// would otherwise succeed. Returns `Some` of an RAII guard which will
/// release the shared access of this thread when dropped, or `None` if the
/// access could not be granted. This method does not provide any
/// guarantees with respect to the ordering of whether contentious readers
/// or writers will acquire the lock first.
///
/// ```
/// let mylock = spin::RwLock::new(0);
/// {
/// match mylock.try_read() {
/// Some(data) => {
/// // The lock is now locked and the data can be read
/// println!("{}", *data);
/// // The lock is dropped
/// },
/// None => (), // no cigar
/// };
/// }
/// ```
#[inline]
pub fn try_read(&self) -> Option<RwLockReadGuard<T>>
{
// Old value, with write bit unset
let old = (!USIZE_MSB) & self.lock.load(Ordering::Relaxed);
let new = old + 1;
debug_assert!(new != (!USIZE_MSB) & (!0));
if self.lock.compare_and_swap(old,
new,
Ordering::SeqCst) == old
{
Some(RwLockReadGuard {
lock: &self.lock,
data: unsafe { & *self.data.get() },
})
} else {
None
}
}
/// Force decrement the reader count.
///
/// This is *extremely* unsafe if there are outstanding `RwLockReadGuard`s
/// live, or if called more times than `read` has been called, but can be
/// useful in FFI contexts where the caller doesn't know how to deal with
/// RAII.
pub unsafe fn force_read_decrement(&self) {
debug_assert!(self.lock.load(Ordering::Relaxed) & (!USIZE_MSB) > 0);
self.lock.fetch_sub(1, Ordering::SeqCst);
}
/// Force unlock exclusive write access.
///
/// This is *extremely* unsafe if there are outstanding `RwLockWriteGuard`s
/// live, or if called when there are current readers, but can be useful in
/// FFI contexts where the caller doesn't know how to deal with RAII.
pub unsafe fn force_write_unlock(&self) {
debug_assert_eq!(self.lock.load(Ordering::Relaxed), USIZE_MSB);
self.lock.store(0, Ordering::Relaxed);
}
/// Lock this rwlock with exclusive write access, blocking the current
/// thread until it can be acquired.
///
/// This function will not return while other writers or other readers
/// currently have access to the lock.
///
/// Returns an RAII guard which will drop the write access of this rwlock
/// when dropped.
///
/// ```
/// let mylock = spin::RwLock::new(0);
/// {
/// let mut data = mylock.write();
/// // The lock is now locked and the data can be written
/// *data += 1;
/// // The lock is dropped
/// }
/// ```
#[inline]
pub fn write<'a>(&'a self) -> RwLockWriteGuard<'a, T>
{
loop
{
// Old value, with write bit unset.
let old = (!USIZE_MSB) & self.lock.load(Ordering::Relaxed);
// Old value, with write bit set.
let new = USIZE_MSB | old;
if self.lock.compare_and_swap(old,
new,
Ordering::SeqCst) == old
{
// Wait for readers to go away, then lock is ours.
while self.lock.load(Ordering::Relaxed) != USIZE_MSB {
cpu_relax();
}
break
}
}
RwLockWriteGuard {
lock: &self.lock,
data: unsafe { &mut *self.data.get() },
}
}
/// Attempt to lock this rwlock with exclusive write access.
///
/// This function does not ever block, and it will return `None` if a call
/// to `write` would otherwise block. If successful, an RAII guard is
/// returned.
///
/// ```
/// let mylock = spin::RwLock::new(0);
/// {
/// match mylock.try_write() {
/// Some(mut data) => {
/// // The lock is now locked and the data can be written
/// *data += 1;
/// // The lock is implicitly dropped
/// },
/// None => (), // no cigar
/// };
/// }
/// ```
#[inline]
pub fn try_write(&self) -> Option<RwLockWriteGuard<T>>
{
if self.lock.compare_and_swap(0,
USIZE_MSB,
Ordering::SeqCst) == 0
{
Some(RwLockWriteGuard {
lock: &self.lock,
data: unsafe { &mut *self.data.get() },
})
} else {
None
}
}
}
impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T>
{
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result
{
match self.try_read()
{
Some(guard) => write!(f, "RwLock {{ data: {:?} }}", &*guard),
None => write!(f, "RwLock {{ <locked> }}"),
}
}
}
impl<T: ?Sized + Default> Default for RwLock<T> {
fn default() -> RwLock<T> {
RwLock::new(Default::default())
}
}
impl<'rwlock, T: ?Sized> Deref for RwLockReadGuard<'rwlock, T> {
type Target = T;
fn deref(&self) -> &T { self.data }
}
impl<'rwlock, T: ?Sized> Deref for RwLockWriteGuard<'rwlock, T> {
type Target = T;
fn deref(&self) -> &T { self.data }
}
impl<'rwlock, T: ?Sized> DerefMut for RwLockWriteGuard<'rwlock, T> {
fn deref_mut(&mut self) -> &mut T { self.data }
}
impl<'rwlock, T: ?Sized> Drop for RwLockReadGuard<'rwlock, T> {
fn drop(&mut self) {
debug_assert!(self.lock.load(Ordering::Relaxed) & (!USIZE_MSB) > 0);
self.lock.fetch_sub(1, Ordering::SeqCst);
}
}
impl<'rwlock, T: ?Sized> Drop for RwLockWriteGuard<'rwlock, T> {
fn drop(&mut self) {
debug_assert_eq!(self.lock.load(Ordering::Relaxed), USIZE_MSB);
self.lock.store(0, Ordering::Relaxed);
}
}
#[cfg(test)]
mod tests {
use std::prelude::v1::*;
use std::sync::Arc;
use std::sync::mpsc::channel;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::thread;
use super::*;
#[derive(Eq, PartialEq, Debug)]
struct NonCopy(i32);
#[test]
fn smoke() {
let l = RwLock::new(());
drop(l.read());
drop(l.write());
drop((l.read(), l.read()));
drop(l.write());
}
// TODO: needs RNG
//#[test]
//fn frob() {
// static R: RwLock = RwLock::new();
// const N: usize = 10;
// const M: usize = 1000;
//
// let (tx, rx) = channel::<()>();
// for _ in 0..N {
// let tx = tx.clone();
// thread::spawn(move|| {
// let mut rng = rand::thread_rng();
// for _ in 0..M {
// if rng.gen_weighted_bool(N) {
// drop(R.write());
// } else {
// drop(R.read());
// }
// }
// drop(tx);
// });
// }
// drop(tx);
// let _ = rx.recv();
// unsafe { R.destroy(); }
//}
#[test]
fn test_rw_arc() {
let arc = Arc::new(RwLock::new(0));
let arc2 = arc.clone();
let (tx, rx) = channel();
thread::spawn(move|| {
let mut lock = arc2.write();
for _ in 0..10 {
let tmp = *lock;
*lock = -1;
thread::yield_now();
*lock = tmp + 1;
}
tx.send(()).unwrap();
});
// Readers try to catch the writer in the act
let mut children = Vec::new();
for _ in 0..5 {
let arc3 = arc.clone();
children.push(thread::spawn(move|| {
let lock = arc3.read();
assert!(*lock >= 0);
}));
}
// Wait for children to pass their asserts
for r in children {
assert!(r.join().is_ok());
}
// Wait for writer to finish
rx.recv().unwrap();
let lock = arc.read();
assert_eq!(*lock, 10);
}
#[test]
fn test_rw_arc_access_in_unwind() {
let arc = Arc::new(RwLock::new(1));
let arc2 = arc.clone();
let _ = thread::spawn(move|| -> () {
struct Unwinder {
i: Arc<RwLock<isize>>,
}
impl Drop for Unwinder {
fn drop(&mut self) {
let mut lock = self.i.write();
*lock += 1;
}
}
let _u = Unwinder { i: arc2 };
panic!();
}).join();
let lock = arc.read();
assert_eq!(*lock, 2);
}
#[test]
fn test_rwlock_unsized() {
let rw: &RwLock<[i32]> = &RwLock::new([1, 2, 3]);
{
let b = &mut *rw.write();
b[0] = 4;
b[2] = 5;
}
let comp: &[i32] = &[4, 2, 5];
assert_eq!(&*rw.read(), comp);
}
#[test]
fn test_rwlock_try_write() {
use std::mem::drop;
let lock = RwLock::new(0isize);
let read_guard = lock.read();
let write_result = lock.try_write();
match write_result {
None => (),
Some(_) => assert!(false, "try_write should not succeed while read_guard is in scope"),
}
drop(read_guard);
}
#[test]
fn test_into_inner() {
let m = RwLock::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 = RwLock::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_force_read_decrement() {
let m = RwLock::new(());
::std::mem::forget(m.read());
::std::mem::forget(m.read());
::std::mem::forget(m.read());
assert!(m.try_write().is_none());
unsafe {
m.force_read_decrement();
m.force_read_decrement();
}
assert!(m.try_write().is_none());
unsafe {
m.force_read_decrement();
}
assert!(m.try_write().is_some());
}
#[test]
fn test_force_write_unlock() {
let m = RwLock::new(());
::std::mem::forget(m.write());
assert!(m.try_read().is_none());
unsafe {
m.force_write_unlock();
}
assert!(m.try_read().is_some());
}
}