pub struct RwLock<T: ?Sized> { /* private fields */ }
Expand description
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
}
Implementations§
Source§impl<T: ?Sized> RwLock<T>
impl<T: ?Sized> RwLock<T>
Sourcepub fn new(value: T) -> RwLock<T>where
T: Sized,
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);
Sourcepub fn with_max_readers(value: T, max_reads: u32) -> RwLock<T>where
T: Sized,
pub fn with_max_readers(value: T, max_reads: u32) -> RwLock<T>where
T: Sized,
Sourcepub const fn const_new(value: T) -> RwLock<T>where
T: Sized,
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);
Sourcepub const fn const_with_max_readers(value: T, max_reads: u32) -> RwLock<T>where
T: Sized,
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);
Sourcepub async fn read(&self) -> RwLockReadGuard<'_, T>
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);
}
Sourcepub fn blocking_read(&self) -> RwLockReadGuard<'_, T>
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 insidespawn_blocking()
(orblock_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());
}
Sourcepub async fn read_owned(self: Arc<Self>) -> OwnedRwLockReadGuard<T>
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);
}
Sourcepub fn try_read(&self) -> Result<RwLockReadGuard<'_, T>, TryLockError>
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);
}
Sourcepub fn try_read_owned(
self: Arc<Self>,
) -> Result<OwnedRwLockReadGuard<T>, TryLockError>
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);
}
Sourcepub async fn write(&self) -> RwLockWriteGuard<'_, T>
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;
}
Sourcepub fn blocking_write(&self) -> RwLockWriteGuard<'_, T>
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 insidespawn_blocking()
(orblock_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);
}
Sourcepub async fn write_owned(self: Arc<Self>) -> OwnedRwLockWriteGuard<T>
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;
}
Sourcepub fn try_write(&self) -> Result<RwLockWriteGuard<'_, T>, TryLockError>
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());
}
Sourcepub fn try_write_owned(
self: Arc<Self>,
) -> Result<OwnedRwLockWriteGuard<T>, TryLockError>
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());
}
Sourcepub fn get_mut(&mut self) -> &mut T
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;
}
Sourcepub fn into_inner(self) -> Twhere
T: Sized,
pub fn into_inner(self) -> Twhere
T: Sized,
Consumes the lock, returning the underlying data.