Struct tokio::sync::RwLock

pub struct RwLock<T: ?Sized> {
    mr: u32,
    s: Semaphore,
    c: UnsafeCell<T>,
}

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 (and 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
}

Fields

mr: u32

s: Semaphore

c: UnsafeCell<T>

Implementations

impl<T: ?Sized> RwLock<T>

pub fn new(value: T) -> RwLock<T>

where T: Sized,

Creates a new instance of an RwLock<T> which is unlocked.

Examples

use tokio::sync::RwLock;

let lock = RwLock::new(5);

pub fn with_max_readers(value: T, max_reads: u32) -> RwLock<T>

where T: Sized,

Creates a new instance of an RwLock<T> which is unlocked and allows a maximum of max_reads concurrent readers.

Examples

use tokio::sync::RwLock;

let lock = RwLock::with_max_readers(5, 1024);

Panics

Panics if max_reads is more than u32::MAX >> 3.

pub const fn const_new(value: T) -> RwLock<T>

where T: Sized,

Creates a new instance of an RwLock<T> which is unlocked.

When using the tracing unstable feature, a RwLock created with const_new will not be instrumented. As such, it will not be visible in tokio-console. Instead, RwLock::new should be used to create an instrumented object if that is needed.

Examples

use tokio::sync::RwLock;

static LOCK: RwLock<i32> = RwLock::const_new(5);

pub const fn const_with_max_readers(value: T, max_reads: u32) -> RwLock<T>

where T: Sized,

Creates a new instance of an RwLock<T> which is unlocked and allows a maximum of max_reads concurrent readers.

Examples

use tokio::sync::RwLock;

static LOCK: RwLock<i32> = RwLock::const_with_max_readers(5, 1024);

pub async fn read(&self) -> RwLockReadGuard<'_, T>

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 writers which hold the lock. There may be other readers inside the lock when the task resumes.

Note that under the priority policy of RwLock, read locks are not granted until prior write locks, to prevent starvation. Therefore deadlock may occur if a read lock is held by the current task, a write lock attempt is made, and then a subsequent read lock attempt is made by the current task.

Returns an RAII guard which will drop this read access of the RwLock when dropped.

Cancel safety

This method uses a queue to fairly distribute locks in the order they were requested. Cancelling a call to read makes you lose your place in the queue.

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 panicked");

    // Drop the guard after the spawned task finishes.
    drop(n);
}

pub fn blocking_read(&self) -> RwLockReadGuard<'_, T>

Blockingly locks this RwLock with shared read access.

This method is intended for use cases where you need to use this rwlock in asynchronous code as well as in synchronous code.

Returns an RAII guard which will drop the read access of this RwLock when dropped.

Panics

This function panics if called within an asynchronous execution context.

• If you find yourself in an asynchronous execution context and needing to call some (synchronous) function which performs one of these blocking_ operations, then consider wrapping that call inside spawn_blocking() (or block_in_place()).

Examples

use std::sync::Arc;
use tokio::sync::RwLock;

#[tokio::main]
async fn main() {
    let rwlock = Arc::new(RwLock::new(1));
    let mut write_lock = rwlock.write().await;

    let blocking_task = tokio::task::spawn_blocking({
        let rwlock = Arc::clone(&rwlock);
        move || {
            // This shall block until the `write_lock` is released.
            let read_lock = rwlock.blocking_read();
            assert_eq!(*read_lock, 0);
        }
    });

    *write_lock -= 1;
    drop(write_lock); // release the lock.

    // Await the completion of the blocking task.
    blocking_task.await.unwrap();

    // Assert uncontended.
    assert!(rwlock.try_write().is_ok());
}

pub async fn read_owned(self: Arc<Self>) -> OwnedRwLockReadGuard<T>

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 writers which hold the lock. There may be other readers inside the lock when the task resumes.

This method is identical to RwLock::read, except that the returned guard references the RwLock with an Arc rather than by borrowing it. Therefore, the RwLock must be wrapped in an Arc to call this method, and the guard will live for the 'static lifetime, as it keeps the RwLock alive by holding an Arc.

Note that under the priority policy of RwLock, read locks are not granted until prior write locks, to prevent starvation. Therefore deadlock may occur if a read lock is held by the current task, a write lock attempt is made, and then a subsequent read lock attempt is made by the current task.

Returns an RAII guard which will drop this read access of the RwLock when dropped.

Cancel safety

This method uses a queue to fairly distribute locks in the order they were requested. Cancelling a call to read_owned makes you lose your place in the queue.

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_owned().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_owned().await;
        assert_eq!(*r, 1);
    }).await.expect("The spawned task has panicked");

    // Drop the guard after the spawned task finishes.
    drop(n);
}

pub fn try_read(&self) -> Result<RwLockReadGuard<'_, T>, TryLockError>

Attempts to acquire this RwLock with shared read access.

If the access couldn’t be acquired immediately, returns TryLockError. Otherwise, an RAII guard is returned which will release read access when dropped.

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 v = lock.try_read().unwrap();
    assert_eq!(*v, 1);

    tokio::spawn(async move {
        // While main has an active read lock, we acquire one too.
        let n = c_lock.read().await;
        assert_eq!(*n, 1);
    }).await.expect("The spawned task has panicked");

    // Drop the guard when spawned task finishes.
    drop(v);
}

pub fn try_read_owned( self: Arc<Self>, ) -> Result<OwnedRwLockReadGuard<T>, TryLockError>

Attempts to acquire this RwLock with shared read access.

If the access couldn’t be acquired immediately, returns TryLockError. Otherwise, an RAII guard is returned which will release read access when dropped.

This method is identical to RwLock::try_read, except that the returned guard references the RwLock with an Arc rather than by borrowing it. Therefore, the RwLock must be wrapped in an Arc to call this method, and the guard will live for the 'static lifetime, as it keeps the RwLock alive by holding an Arc.

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 v = lock.try_read_owned().unwrap();
    assert_eq!(*v, 1);

    tokio::spawn(async move {
        // While main has an active read lock, we acquire one too.
        let n = c_lock.read_owned().await;
        assert_eq!(*n, 1);
    }).await.expect("The spawned task has panicked");

    // Drop the guard when spawned task finishes.
    drop(v);
}

pub async fn write(&self) -> RwLockWriteGuard<'_, T>

Locks this RwLock with exclusive write access, causing the current task to yield until the lock has been acquired.

The calling task will yield while other writers or readers currently have access to the lock.

Returns an RAII guard which will drop the write access of this RwLock when dropped.

Cancel safety

This method uses a queue to fairly distribute locks in the order they were requested. Cancelling a call to write makes you lose your place in the queue.

Examples

use tokio::sync::RwLock;

#[tokio::main]
async fn main() {
  let lock = RwLock::new(1);

  let mut n = lock.write().await;
  *n = 2;
}

pub fn blocking_write(&self) -> RwLockWriteGuard<'_, T>

Blockingly locks this RwLock with exclusive write access.

This method is intended for use cases where you need to use this rwlock in asynchronous code as well as in synchronous code.

Returns an RAII guard which will drop the write access of this RwLock when dropped.

Panics

This function panics if called within an asynchronous execution context.

• If you find yourself in an asynchronous execution context and needing to call some (synchronous) function which performs one of these blocking_ operations, then consider wrapping that call inside spawn_blocking() (or block_in_place()).

Examples

use std::sync::Arc;
use tokio::{sync::RwLock};

#[tokio::main]
async fn main() {
    let rwlock =  Arc::new(RwLock::new(1));
    let read_lock = rwlock.read().await;

    let blocking_task = tokio::task::spawn_blocking({
        let rwlock = Arc::clone(&rwlock);
        move || {
            // This shall block until the `read_lock` is released.
            let mut write_lock = rwlock.blocking_write();
            *write_lock = 2;
        }
    });

    assert_eq!(*read_lock, 1);
    // Release the last outstanding read lock.
    drop(read_lock);

    // Await the completion of the blocking task.
    blocking_task.await.unwrap();

    // Assert uncontended.
    let read_lock = rwlock.try_read().unwrap();
    assert_eq!(*read_lock, 2);
}

pub async fn write_owned(self: Arc<Self>) -> OwnedRwLockWriteGuard<T>

Locks this RwLock with exclusive write access, causing the current task to yield until the lock has been acquired.

The calling task will yield while other writers or readers currently have access to the lock.

This method is identical to RwLock::write, except that the returned guard references the RwLock with an Arc rather than by borrowing it. Therefore, the RwLock must be wrapped in an Arc to call this method, and the guard will live for the 'static lifetime, as it keeps the RwLock alive by holding an Arc.

Returns an RAII guard which will drop the write access of this RwLock when dropped.

Cancel safety

This method uses a queue to fairly distribute locks in the order they were requested. Cancelling a call to write_owned makes you lose your place in the queue.

Examples

use std::sync::Arc;
use tokio::sync::RwLock;

#[tokio::main]
async fn main() {
  let lock = Arc::new(RwLock::new(1));

  let mut n = lock.write_owned().await;
  *n = 2;
}

pub fn try_write(&self) -> Result<RwLockWriteGuard<'_, T>, TryLockError>

Attempts to acquire this RwLock with exclusive write access.

If the access couldn’t be acquired immediately, returns TryLockError. Otherwise, an RAII guard is returned which will release write access when dropped.

Examples

use tokio::sync::RwLock;

#[tokio::main]
async fn main() {
    let rw = RwLock::new(1);

    let v = rw.read().await;
    assert_eq!(*v, 1);

    assert!(rw.try_write().is_err());
}

pub fn try_write_owned( self: Arc<Self>, ) -> Result<OwnedRwLockWriteGuard<T>, TryLockError>

Attempts to acquire this RwLock with exclusive write access.

If the access couldn’t be acquired immediately, returns TryLockError. Otherwise, an RAII guard is returned which will release write access when dropped.

This method is identical to RwLock::try_write, except that the returned guard references the RwLock with an Arc rather than by borrowing it. Therefore, the RwLock must be wrapped in an Arc to call this method, and the guard will live for the 'static lifetime, as it keeps the RwLock alive by holding an Arc.

Examples

use std::sync::Arc;
use tokio::sync::RwLock;

#[tokio::main]
async fn main() {
    let rw = Arc::new(RwLock::new(1));

    let v = Arc::clone(&rw).read_owned().await;
    assert_eq!(*v, 1);

    assert!(rw.try_write_owned().is_err());
}

pub fn get_mut(&mut self) -> &mut T

Returns a mutable reference to the underlying data.

Since this call borrows the RwLock mutably, no actual locking needs to take place – the mutable borrow statically guarantees no locks exist.

Examples

use tokio::sync::RwLock;

fn main() {
    let mut lock = RwLock::new(1);

    let n = lock.get_mut();
    *n = 2;
}

pub fn into_inner(self) -> T

where T: Sized,

Consumes the lock, returning the underlying data.

Trait Implementations

impl<T> Debug for RwLock<T>

where T: Debug + ?Sized,

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T> Default for RwLock<T>

where T: Default + ?Sized,

fn default() -> Self

Returns the “default value” for a type.

impl<T> From<T> for RwLock<T>

fn from(s: T) -> Self

Converts to this type from the input type.

impl<T> Send for RwLock<T>

where T: ?Sized + Send,

impl<T> Sync for RwLock<T>

where T: ?Sized + Send + Sync,