Revision control
Copy as Markdown
Other Tools
// Copyright 2016 Amanieu d'Antras
//
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// copied, modified, or distributed except according to those terms.
use crate::raw_rwlock::RawRwLock;
use lock_api;
/// 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).
///
/// This lock uses a task-fair locking policy which avoids both reader and
/// writer starvation. This means that readers trying to acquire the lock will
/// block even if the lock is unlocked when there are writers waiting to acquire
/// the lock. Because of this, attempts to recursively acquire a read lock
/// within a single thread may result in a deadlock.
///
/// The type parameter `T` represents the data that this lock protects. It is
/// required that `T` satisfies `Send` to be shared across threads 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.
///
/// # Fairness
///
/// A typical unfair lock can often end up in a situation where a single thread
/// quickly acquires and releases the same lock in succession, which can starve
/// other threads waiting to acquire the rwlock. While this improves throughput
/// because it doesn't force a context switch when a thread tries to re-acquire
/// a rwlock it has just released, this can starve other threads.
///
/// to ensure that the lock will be fair on average without sacrificing
/// throughput. This is done by forcing a fair unlock on average every 0.5ms,
/// which will force the lock to go to the next thread waiting for the rwlock.
///
/// Additionally, any critical section longer than 1ms will always use a fair
/// unlock, which has a negligible impact on throughput considering the length
/// of the critical section.
///
/// You can also force a fair unlock by calling `RwLockReadGuard::unlock_fair`
/// or `RwLockWriteGuard::unlock_fair` when unlocking a mutex instead of simply
/// dropping the guard.
///
/// # Differences from the standard library `RwLock`
///
/// - Supports atomically downgrading a write lock into a read lock.
/// - Task-fair locking policy instead of an unspecified platform default.
/// - No poisoning, the lock is released normally on panic.
/// - Only requires 1 word of space, whereas the standard library boxes the
/// `RwLock` due to platform limitations.
/// - Can be statically constructed.
/// - Does not require any drop glue when dropped.
/// - Inline fast path for the uncontended case.
/// - Efficient handling of micro-contention using adaptive spinning.
/// - Allows raw locking & unlocking without a guard.
/// - Supports eventual fairness so that the rwlock is fair on average.
/// - Optionally allows making the rwlock fair by calling
/// `RwLockReadGuard::unlock_fair` and `RwLockWriteGuard::unlock_fair`.
///
/// # Examples
///
/// ```
/// use parking_lot::RwLock;
///
/// let lock = 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 type RwLock<T> = lock_api::RwLock<RawRwLock, T>;
/// Creates a new instance of an `RwLock<T>` which is unlocked.
///
/// This allows creating a `RwLock<T>` in a constant context on stable Rust.
pub const fn const_rwlock<T>(val: T) -> RwLock<T> {
RwLock::const_new(<RawRwLock as lock_api::RawRwLock>::INIT, val)
}
/// RAII structure used to release the shared read access of a lock when
/// dropped.
pub type RwLockReadGuard<'a, T> = lock_api::RwLockReadGuard<'a, RawRwLock, T>;
/// RAII structure used to release the exclusive write access of a lock when
/// dropped.
pub type RwLockWriteGuard<'a, T> = lock_api::RwLockWriteGuard<'a, RawRwLock, T>;
/// An RAII read lock guard returned by `RwLockReadGuard::map`, which can point to a
/// subfield of the protected data.
///
/// The main difference between `MappedRwLockReadGuard` and `RwLockReadGuard` is that the
/// former doesn't support temporarily unlocking and re-locking, since that
/// could introduce soundness issues if the locked object is modified by another
/// thread.
pub type MappedRwLockReadGuard<'a, T> = lock_api::MappedRwLockReadGuard<'a, RawRwLock, T>;
/// An RAII write lock guard returned by `RwLockWriteGuard::map`, which can point to a
/// subfield of the protected data.
///
/// The main difference between `MappedRwLockWriteGuard` and `RwLockWriteGuard` is that the
/// former doesn't support temporarily unlocking and re-locking, since that
/// could introduce soundness issues if the locked object is modified by another
/// thread.
pub type MappedRwLockWriteGuard<'a, T> = lock_api::MappedRwLockWriteGuard<'a, RawRwLock, T>;
/// RAII structure used to release the upgradable read access of a lock when
/// dropped.
pub type RwLockUpgradableReadGuard<'a, T> = lock_api::RwLockUpgradableReadGuard<'a, RawRwLock, T>;
#[cfg(test)]
mod tests {
use crate::{RwLock, RwLockUpgradableReadGuard, RwLockWriteGuard};
use rand::Rng;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::sync::mpsc::channel;
use std::sync::Arc;
use std::thread;
use std::time::Duration;
#[cfg(feature = "serde")]
use bincode::{deserialize, serialize};
#[derive(Eq, PartialEq, Debug)]
struct NonCopy(i32);
#[test]
fn smoke() {
let l = RwLock::new(());
drop(l.read());
drop(l.write());
drop(l.upgradable_read());
drop((l.read(), l.read()));
drop((l.read(), l.upgradable_read()));
drop(l.write());
}
#[test]
fn frob() {
const N: u32 = 10;
const M: u32 = 1000;
let r = Arc::new(RwLock::new(()));
let (tx, rx) = channel::<()>();
for _ in 0..N {
let tx = tx.clone();
let r = r.clone();
thread::spawn(move || {
let mut rng = rand::thread_rng();
for _ in 0..M {
if rng.gen_bool(1.0 / N as f64) {
drop(r.write());
} else {
drop(r.read());
}
}
drop(tx);
});
}
drop(tx);
let _ = rx.recv();
}
#[test]
fn test_rw_arc_no_poison_wr() {
let arc = Arc::new(RwLock::new(1));
let arc2 = arc.clone();
let _: Result<(), _> = thread::spawn(move || {
let _lock = arc2.write();
panic!();
})
.join();
let lock = arc.read();
assert_eq!(*lock, 1);
}
#[test]
fn test_rw_arc_no_poison_ww() {
let arc = Arc::new(RwLock::new(1));
let arc2 = arc.clone();
let _: Result<(), _> = thread::spawn(move || {
let _lock = arc2.write();
panic!();
})
.join();
let lock = arc.write();
assert_eq!(*lock, 1);
}
#[test]
fn test_rw_arc_no_poison_rr() {
let arc = Arc::new(RwLock::new(1));
let arc2 = arc.clone();
let _: Result<(), _> = thread::spawn(move || {
let _lock = arc2.read();
panic!();
})
.join();
let lock = arc.read();
assert_eq!(*lock, 1);
}
#[test]
fn test_rw_arc_no_poison_rw() {
let arc = Arc::new(RwLock::new(1));
let arc2 = arc.clone();
let _: Result<(), _> = thread::spawn(move || {
let _lock = arc2.read();
panic!()
})
.join();
let lock = arc.write();
assert_eq!(*lock, 1);
}
#[test]
fn test_ruw_arc() {
let arc = Arc::new(RwLock::new(0));
let arc2 = arc.clone();
let (tx, rx) = channel();
thread::spawn(move || {
for _ in 0..10 {
let mut lock = arc2.write();
let tmp = *lock;
*lock = -1;
thread::yield_now();
*lock = tmp + 1;
}
tx.send(()).unwrap();
});
let mut children = Vec::new();
// Upgradable readers try to catch the writer in the act and also
// try to touch the value
for _ in 0..5 {
let arc3 = arc.clone();
children.push(thread::spawn(move || {
let lock = arc3.upgradable_read();
let tmp = *lock;
assert!(tmp >= 0);
thread::yield_now();
let mut lock = RwLockUpgradableReadGuard::upgrade(lock);
assert_eq!(tmp, *lock);
*lock = -1;
thread::yield_now();
*lock = tmp + 1;
}));
}
// Readers try to catch the writers in the act
for _ in 0..5 {
let arc4 = arc.clone();
children.push(thread::spawn(move || {
let lock = arc4.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, 15);
}
#[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_read() {
let lock = RwLock::new(0isize);
{
let read_guard = lock.read();
let read_result = lock.try_read();
assert!(
read_result.is_some(),
"try_read should succeed while read_guard is in scope"
);
drop(read_guard);
}
{
let upgrade_guard = lock.upgradable_read();
let read_result = lock.try_read();
assert!(
read_result.is_some(),
"try_read should succeed while upgrade_guard is in scope"
);
drop(upgrade_guard);
}
{
let write_guard = lock.write();
let read_result = lock.try_read();
assert!(
read_result.is_none(),
"try_read should fail while write_guard is in scope"
);
drop(write_guard);
}
}
#[test]
fn test_rwlock_try_write() {
let lock = RwLock::new(0isize);
{
let read_guard = lock.read();
let write_result = lock.try_write();
assert!(
write_result.is_none(),
"try_write should fail while read_guard is in scope"
);
assert!(lock.is_locked());
assert!(!lock.is_locked_exclusive());
drop(read_guard);
}
{
let upgrade_guard = lock.upgradable_read();
let write_result = lock.try_write();
assert!(
write_result.is_none(),
"try_write should fail while upgrade_guard is in scope"
);
assert!(lock.is_locked());
assert!(!lock.is_locked_exclusive());
drop(upgrade_guard);
}
{
let write_guard = lock.write();
let write_result = lock.try_write();
assert!(
write_result.is_none(),
"try_write should fail while write_guard is in scope"
);
assert!(lock.is_locked());
assert!(lock.is_locked_exclusive());
drop(write_guard);
}
}
#[test]
fn test_rwlock_try_upgrade() {
let lock = RwLock::new(0isize);
{
let read_guard = lock.read();
let upgrade_result = lock.try_upgradable_read();
assert!(
upgrade_result.is_some(),
"try_upgradable_read should succeed while read_guard is in scope"
);
drop(read_guard);
}
{
let upgrade_guard = lock.upgradable_read();
let upgrade_result = lock.try_upgradable_read();
assert!(
upgrade_result.is_none(),
"try_upgradable_read should fail while upgrade_guard is in scope"
);
drop(upgrade_guard);
}
{
let write_guard = lock.write();
let upgrade_result = lock.try_upgradable_read();
assert!(
upgrade_result.is_none(),
"try_upgradable should fail while write_guard is in scope"
);
drop(write_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_get_mut() {
let mut m = RwLock::new(NonCopy(10));
*m.get_mut() = NonCopy(20);
assert_eq!(m.into_inner(), NonCopy(20));
}
#[test]
fn test_rwlockguard_sync() {
fn sync<T: Sync>(_: T) {}
let rwlock = RwLock::new(());
sync(rwlock.read());
sync(rwlock.write());
}
#[test]
fn test_rwlock_downgrade() {
let x = Arc::new(RwLock::new(0));
let mut handles = Vec::new();
for _ in 0..8 {
let x = x.clone();
handles.push(thread::spawn(move || {
for _ in 0..100 {
let mut writer = x.write();
*writer += 1;
let cur_val = *writer;
let reader = RwLockWriteGuard::downgrade(writer);
assert_eq!(cur_val, *reader);
}
}));
}
for handle in handles {
handle.join().unwrap()
}
assert_eq!(*x.read(), 800);
}
#[test]
fn test_rwlock_recursive() {
let arc = Arc::new(RwLock::new(1));
let arc2 = arc.clone();
let lock1 = arc.read();
let t = thread::spawn(move || {
let _lock = arc2.write();
});
if cfg!(not(all(target_env = "sgx", target_vendor = "fortanix"))) {
thread::sleep(Duration::from_millis(100));
} else {
for _ in 0..100 {
thread::yield_now();
}
}
// A normal read would block here since there is a pending writer
let lock2 = arc.read_recursive();
// Unblock the thread and join it.
drop(lock1);
drop(lock2);
t.join().unwrap();
}
#[test]
fn test_rwlock_debug() {
let x = RwLock::new(vec![0u8, 10]);
assert_eq!(format!("{:?}", x), "RwLock { data: [0, 10] }");
let _lock = x.write();
assert_eq!(format!("{:?}", x), "RwLock { data: <locked> }");
}
#[test]
fn test_clone() {
let rwlock = RwLock::new(Arc::new(1));
let a = rwlock.read_recursive();
let b = a.clone();
assert_eq!(Arc::strong_count(&b), 2);
}
#[cfg(feature = "serde")]
#[test]
fn test_serde() {
let contents: Vec<u8> = vec![0, 1, 2];
let mutex = RwLock::new(contents.clone());
let serialized = serialize(&mutex).unwrap();
let deserialized: RwLock<Vec<u8>> = deserialize(&serialized).unwrap();
assert_eq!(*(mutex.read()), *(deserialized.read()));
assert_eq!(contents, *(deserialized.read()));
}
#[test]
fn test_issue_203() {
struct Bar(RwLock<()>);
impl Drop for Bar {
fn drop(&mut self) {
let _n = self.0.write();
}
}
thread_local! {
static B: Bar = Bar(RwLock::new(()));
}
thread::spawn(|| {
B.with(|_| ());
let a = RwLock::new(());
let _a = a.read();
})
.join()
.unwrap();
}
#[test]
fn test_rw_write_is_locked() {
let lock = RwLock::new(0isize);
{
let _read_guard = lock.read();
assert!(lock.is_locked());
assert!(!lock.is_locked_exclusive());
}
{
let _write_guard = lock.write();
assert!(lock.is_locked());
assert!(lock.is_locked_exclusive());
}
}
}