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tokio/tokio/src/sync/rwlock.rs
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use crate::sync::batch_semaphore::Semaphore;
use std::cell::UnsafeCell;
use std::fmt;
use std::marker;
use std::mem;
use std::ops;
#[cfg(not(loom))]
const MAX_READS: usize = 32;
#[cfg(loom)]
const MAX_READS: usize = 10;
/// An asynchronous 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).
///
/// In comparison, a [`Mutex`] does not distinguish between readers or writers
/// that acquire the lock, therefore causing any tasks waiting for the lock to
/// become available to yield. An `RwLock` will allow any number of readers to
/// acquire the lock as long as a writer is not holding the lock.
///
/// The priority policy of Tokio's read-write lock is _fair_ (or
/// [_write-preferring_]), in order to ensure that readers cannot starve
/// writers. Fairness is ensured using a first-in, first-out queue for the tasks
/// awaiting the lock; if a task that wishes to acquire the write lock is at the
/// head of the queue, read locks will not be given out until the write lock has
/// been released. This is in contrast to the Rust standard library's
/// `std::sync::RwLock`, where the priority policy is dependent on the
/// operating system's implementation.
///
/// The type parameter `T` represents the data that this lock protects. It is
/// required that `T` satisfies [`Send`] to be shared across threads. The RAII guards
/// returned from the locking methods implement [`Deref`](trait@std::ops::Deref)
/// (and [`DerefMut`](trait@std::ops::DerefMut)
/// for the `write` methods) to allow access to the content of the lock.
///
/// # Examples
///
/// ```
/// use tokio::sync::RwLock;
///
/// #[tokio::main]
/// async fn main() {
/// let lock = RwLock::new(5);
///
/// // many reader locks can be held at once
/// {
/// let r1 = lock.read().await;
/// let r2 = lock.read().await;
/// 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().await;
/// *w += 1;
/// assert_eq!(*w, 6);
/// } // write lock is dropped here
/// }
/// ```
///
/// [`Mutex`]: struct@super::Mutex
/// [`RwLock`]: struct@RwLock
/// [`RwLockReadGuard`]: struct@RwLockReadGuard
/// [`RwLockWriteGuard`]: struct@RwLockWriteGuard
/// [`Send`]: trait@std::marker::Send
/// [_write-preferring_]: https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock#Priority_policies
#[derive(Debug)]
pub struct RwLock<T: ?Sized> {
//semaphore to coordinate read and write access to T
s: Semaphore,
//inner data T
c: UnsafeCell<T>,
}
/// RAII structure used to release the shared read access of a lock when
/// dropped.
///
/// This structure is created by the [`read`] method on
/// [`RwLock`].
///
/// [`read`]: method@RwLock::read
/// [`RwLock`]: struct@RwLock
pub struct RwLockReadGuard<'a, T: ?Sized> {
s: &'a Semaphore,
data: *const T,
marker: marker::PhantomData<&'a T>,
}
impl<'a, T> RwLockReadGuard<'a, T> {
/// Make a new `RwLockReadGuard` for a component of the locked data.
///
/// This operation cannot fail as the `RwLockReadGuard` passed in already
/// locked the data.
///
/// This is an associated function that needs to be
/// used as `RwLockReadGuard::map(...)`. A method would interfere with
/// methods of the same name on the contents of the locked data.
///
/// This is an asynchronous version of [`RwLockReadGuard::map`] from the
/// [`parking_lot` crate].
///
/// [`RwLockReadGuard::map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockReadGuard.html#method.map
/// [`parking_lot` crate]: https://crates.io/crates/parking_lot
///
/// # Examples
///
/// ```
/// use tokio::sync::{RwLock, RwLockReadGuard};
///
/// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
/// struct Foo(u32);
///
/// # #[tokio::main]
/// # async fn main() {
/// let lock = RwLock::new(Foo(1));
///
/// let guard = lock.read().await;
/// let guard = RwLockReadGuard::map(guard, |f| &f.0);
///
/// assert_eq!(1, *guard);
/// # }
/// ```
#[inline]
pub fn map<F, U: ?Sized>(this: Self, f: F) -> RwLockReadGuard<'a, U>
where
F: FnOnce(&T) -> &U,
{
let data = f(&*this) as *const U;
let s = this.s;
// NB: Forget to avoid drop impl from being called.
mem::forget(this);
RwLockReadGuard {
s,
data,
marker: marker::PhantomData,
}
}
/// Attempts to make a new [`RwLockReadGuard`] for a component of the
/// locked data. The original guard is returned if the closure returns
/// `None`.
///
/// This operation cannot fail as the `RwLockReadGuard` passed in already
/// locked the data.
///
/// This is an associated function that needs to be used as
/// `RwLockReadGuard::try_map(..)`. A method would interfere with methods of the
/// same name on the contents of the locked data.
///
/// This is an asynchronous version of [`RwLockReadGuard::try_map`] from the
/// [`parking_lot` crate].
///
/// [`RwLockReadGuard::try_map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockReadGuard.html#method.try_map
/// [`parking_lot` crate]: https://crates.io/crates/parking_lot
///
/// # Examples
///
/// ```
/// use tokio::sync::{RwLock, RwLockReadGuard};
///
/// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
/// struct Foo(u32);
///
/// # #[tokio::main]
/// # async fn main() {
/// let lock = RwLock::new(Foo(1));
///
/// let guard = lock.read().await;
/// let guard = RwLockReadGuard::try_map(guard, |f| Some(&f.0)).expect("should not fail");
///
/// assert_eq!(1, *guard);
/// # }
/// ```
#[inline]
pub fn try_map<F, U: ?Sized>(this: Self, f: F) -> Result<RwLockReadGuard<'a, U>, Self>
where
F: FnOnce(&T) -> Option<&U>,
{
let data = match f(&*this) {
Some(data) => data as *const U,
None => return Err(this),
};
let s = this.s;
// NB: Forget to avoid drop impl from being called.
mem::forget(this);
Ok(RwLockReadGuard {
s,
data,
marker: marker::PhantomData,
})
}
}
impl<'a, T: ?Sized> fmt::Debug for RwLockReadGuard<'a, T>
where
T: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
impl<'a, T: ?Sized> fmt::Display for RwLockReadGuard<'a, T>
where
T: fmt::Display,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt(&**self, f)
}
}
impl<'a, T: ?Sized> Drop for RwLockReadGuard<'a, T> {
fn drop(&mut self) {
self.s.release(1);
}
}
/// RAII structure used to release the exclusive write access of a lock when
/// dropped.
///
/// This structure is created by the [`write`] and method
/// on [`RwLock`].
///
/// [`write`]: method@RwLock::write
/// [`RwLock`]: struct@RwLock
pub struct RwLockWriteGuard<'a, T: ?Sized> {
s: &'a Semaphore,
data: *mut T,
marker: marker::PhantomData<&'a mut T>,
}
impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
/// Make a new `RwLockWriteGuard` for a component of the locked data.
///
/// This operation cannot fail as the `RwLockWriteGuard` passed in already
/// locked the data.
///
/// This is an associated function that needs to be used as
/// `RwLockWriteGuard::map(..)`. A method would interfere with methods of
/// the same name on the contents of the locked data.
///
/// This is an asynchronous version of [`RwLockWriteGuard::map`] from the
/// [`parking_lot` crate].
///
/// [`RwLockWriteGuard::map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockWriteGuard.html#method.map
/// [`parking_lot` crate]: https://crates.io/crates/parking_lot
///
/// # Examples
///
/// ```
/// use tokio::sync::{RwLock, RwLockWriteGuard};
///
/// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
/// struct Foo(u32);
///
/// # #[tokio::main]
/// # async fn main() {
/// let lock = RwLock::new(Foo(1));
///
/// {
/// let mut mapped = RwLockWriteGuard::map(lock.write().await, |f| &mut f.0);
/// *mapped = 2;
/// }
///
/// assert_eq!(Foo(2), *lock.read().await);
/// # }
/// ```
#[inline]
pub fn map<F, U: ?Sized>(mut this: Self, f: F) -> RwLockWriteGuard<'a, U>
where
F: FnOnce(&mut T) -> &mut U,
{
let data = f(&mut *this) as *mut U;
let s = this.s;
// NB: Forget to avoid drop impl from being called.
mem::forget(this);
RwLockWriteGuard {
s,
data,
marker: marker::PhantomData,
}
}
/// Attempts to make a new [`RwLockWriteGuard`] for a component of
/// the locked data. The original guard is returned if the closure returns
/// `None`.
///
/// This operation cannot fail as the `RwLockWriteGuard` passed in already
/// locked the data.
///
/// This is an associated function that needs to be
/// used as `RwLockWriteGuard::try_map(...)`. A method would interfere with
/// methods of the same name on the contents of the locked data.
///
/// This is an asynchronous version of [`RwLockWriteGuard::try_map`] from
/// the [`parking_lot` crate].
///
/// [`RwLockWriteGuard::try_map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockWriteGuard.html#method.try_map
/// [`parking_lot` crate]: https://crates.io/crates/parking_lot
///
/// # Examples
///
/// ```
/// use tokio::sync::{RwLock, RwLockWriteGuard};
///
/// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
/// struct Foo(u32);
///
/// # #[tokio::main]
/// # async fn main() {
/// let lock = RwLock::new(Foo(1));
///
/// {
/// let guard = lock.write().await;
/// let mut guard = RwLockWriteGuard::try_map(guard, |f| Some(&mut f.0)).expect("should not fail");
/// *guard = 2;
/// }
///
/// assert_eq!(Foo(2), *lock.read().await);
/// # }
/// ```
#[inline]
pub fn try_map<F, U: ?Sized>(mut this: Self, f: F) -> Result<RwLockWriteGuard<'a, U>, Self>
where
F: FnOnce(&mut T) -> Option<&mut U>,
{
let data = match f(&mut *this) {
Some(data) => data as *mut U,
None => return Err(this),
};
let s = this.s;
// NB: Forget to avoid drop impl from being called.
mem::forget(this);
Ok(RwLockWriteGuard {
s,
data,
marker: marker::PhantomData,
})
}
/// Atomically downgrades a write lock into a read lock without allowing
/// any writers to take exclusive access of the lock in the meantime.
///
/// **Note:** This won't *necessarily* allow any additional readers to acquire
/// locks, since [`RwLock`] is fair and it is possible that a writer is next
/// in line.
///
/// Returns an RAII guard which will drop the read access of this rwlock
/// when dropped.
///
/// # Examples
///
/// ```
/// # use tokio::sync::RwLock;
/// # use std::sync::Arc;
/// #
/// # #[tokio::main]
/// # async fn main() {
/// let lock = Arc::new(RwLock::new(1));
///
/// let n = lock.write().await;
///
/// let cloned_lock = lock.clone();
/// let handle = tokio::spawn(async move {
/// *cloned_lock.write().await = 2;
/// });
///
/// let n = n.downgrade();
/// assert_eq!(*n, 1, "downgrade is atomic");
///
/// assert_eq!(*lock.read().await, 1, "additional readers can obtain locks");
///
/// drop(n);
/// handle.await.unwrap();
/// assert_eq!(*lock.read().await, 2, "second writer obtained write lock");
/// # }
/// ```
///
/// [`RwLock`]: struct@RwLock
pub fn downgrade(self) -> RwLockReadGuard<'a, T> {
let RwLockWriteGuard { s, data, .. } = self;
// Release all but one of the permits held by the write guard
s.release(MAX_READS - 1);
RwLockReadGuard {
s,
data,
marker: marker::PhantomData,
}
}
}
impl<'a, T: ?Sized> fmt::Debug for RwLockWriteGuard<'a, T>
where
T: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
impl<'a, T: ?Sized> fmt::Display for RwLockWriteGuard<'a, T>
where
T: fmt::Display,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt(&**self, f)
}
}
impl<'a, T: ?Sized> Drop for RwLockWriteGuard<'a, T> {
fn drop(&mut self) {
self.s.release(MAX_READS);
}
}
#[test]
#[cfg(not(loom))]
fn bounds() {
fn check_send<T: Send>() {}
fn check_sync<T: Sync>() {}
fn check_unpin<T: Unpin>() {}
// This has to take a value, since the async fn's return type is unnameable.
fn check_send_sync_val<T: Send + Sync>(_t: T) {}
check_send::<RwLock<u32>>();
check_sync::<RwLock<u32>>();
check_unpin::<RwLock<u32>>();
check_send::<RwLockReadGuard<'_, u32>>();
check_sync::<RwLockReadGuard<'_, u32>>();
check_unpin::<RwLockReadGuard<'_, u32>>();
check_send::<RwLockWriteGuard<'_, u32>>();
check_sync::<RwLockWriteGuard<'_, u32>>();
check_unpin::<RwLockWriteGuard<'_, u32>>();
let rwlock = RwLock::new(0);
check_send_sync_val(rwlock.read());
check_send_sync_val(rwlock.write());
}
// As long as T: Send + Sync, it's fine to send and share RwLock<T> between threads.
// If T were not Send, sending and sharing a RwLock<T> would be bad, since you can access T through
// RwLock<T>.
unsafe impl<T> Send for RwLock<T> where T: ?Sized + Send {}
unsafe impl<T> Sync for RwLock<T> where T: ?Sized + Send + Sync {}
// NB: These impls need to be explicit since we're storing a raw pointer.
// Safety: Stores a raw pointer to `T`, so if `T` is `Sync`, the lock guard over
// `T` is `Send`.
unsafe impl<T> Send for RwLockReadGuard<'_, T> where T: ?Sized + Sync {}
unsafe impl<T> Sync for RwLockReadGuard<'_, T> where T: ?Sized + Send + Sync {}
unsafe impl<T> Sync for RwLockWriteGuard<'_, T> where T: ?Sized + Send + Sync {}
// Safety: Stores a raw pointer to `T`, so if `T` is `Sync`, the lock guard over
// `T` is `Send` - but since this is also provides mutable access, we need to
// make sure that `T` is `Send` since its value can be sent across thread
// boundaries.
unsafe impl<T> Send for RwLockWriteGuard<'_, T> where T: ?Sized + Send + Sync {}
impl<T: ?Sized> RwLock<T> {
/// Creates a new instance of an `RwLock<T>` which is unlocked.
///
/// # Examples
///
/// ```
/// use tokio::sync::RwLock;
///
/// let lock = RwLock::new(5);
/// ```
pub fn new(value: T) -> RwLock<T>
where
T: Sized,
{
RwLock {
c: UnsafeCell::new(value),
s: Semaphore::new(MAX_READS),
}
}
/// Locks this rwlock with shared read access, causing the current task
/// to yield until the lock has been acquired.
///
/// The calling task will yield until there are no more writers which
/// hold the lock. There may be other readers currently inside the lock when
/// this method returns.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
/// use tokio::sync::RwLock;
///
/// #[tokio::main]
/// async fn main() {
/// let lock = Arc::new(RwLock::new(1));
/// let c_lock = lock.clone();
///
/// let n = lock.read().await;
/// assert_eq!(*n, 1);
///
/// tokio::spawn(async move {
/// // While main has an active read lock, we acquire one too.
/// let r = c_lock.read().await;
/// assert_eq!(*r, 1);
/// }).await.expect("The spawned task has paniced");
///
/// // Drop the guard after the spawned task finishes.
/// drop(n);
///}
/// ```
pub async fn read(&self) -> RwLockReadGuard<'_, T> {
self.s.acquire(1).await.unwrap_or_else(|_| {
// The semaphore was closed. but, we never explicitly close it, and we have a
// handle to it through the Arc, which means that this can never happen.
unreachable!()
});
RwLockReadGuard {
s: &self.s,
data: self.c.get(),
marker: marker::PhantomData,
}
}
/// Locks this rwlock with exclusive write access, causing the current task
/// to yield until the lock has been 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.
///
/// # Examples
///
/// ```
/// use tokio::sync::RwLock;
///
/// #[tokio::main]
/// async fn main() {
/// let lock = RwLock::new(1);
///
/// let mut n = lock.write().await;
/// *n = 2;
///}
/// ```
pub async fn write(&self) -> RwLockWriteGuard<'_, T> {
self.s.acquire(MAX_READS as u32).await.unwrap_or_else(|_| {
// The semaphore was closed. but, we never explicitly close it, and we have a
// handle to it through the Arc, which means that this can never happen.
unreachable!()
});
RwLockWriteGuard {
s: &self.s,
data: self.c.get(),
marker: marker::PhantomData,
}
}
/// Consumes the lock, returning the underlying data.
pub fn into_inner(self) -> T
where
T: Sized,
{
self.c.into_inner()
}
}
impl<T: ?Sized> ops::Deref for RwLockReadGuard<'_, T> {
type Target = T;
fn deref(&self) -> &T {
unsafe { &*self.data }
}
}
impl<T: ?Sized> ops::Deref for RwLockWriteGuard<'_, T> {
type Target = T;
fn deref(&self) -> &T {
unsafe { &*self.data }
}
}
impl<T: ?Sized> ops::DerefMut for RwLockWriteGuard<'_, T> {
fn deref_mut(&mut self) -> &mut T {
unsafe { &mut *self.data }
}
}
impl<T> From<T> for RwLock<T> {
fn from(s: T) -> Self {
Self::new(s)
}
}
impl<T: ?Sized> Default for RwLock<T>
where
T: Default,
{
fn default() -> Self {
Self::new(T::default())
}
}
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