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-rw-r--r--src/sync/rwlock.rs431
1 files changed, 394 insertions, 37 deletions
diff --git a/src/sync/rwlock.rs b/src/sync/rwlock.rs
index 3d2a2f7..a84c4c1 100644
--- a/src/sync/rwlock.rs
+++ b/src/sync/rwlock.rs
@@ -1,5 +1,8 @@
-use crate::sync::batch_semaphore::{AcquireError, Semaphore};
+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))]
@@ -8,7 +11,7 @@ const MAX_READS: usize = 32;
#[cfg(loom)]
const MAX_READS: usize = 10;
-/// An asynchronous reader-writer lock
+/// 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
@@ -83,10 +86,140 @@ pub struct RwLock<T: ?Sized> {
/// [`RwLock`].
///
/// [`read`]: method@RwLock::read
-#[derive(Debug)]
+/// [`RwLock`]: struct@RwLock
pub struct RwLockReadGuard<'a, T: ?Sized> {
- permit: ReleasingPermit<'a, T>,
- lock: &'a RwLock<T>,
+ 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
@@ -97,32 +230,195 @@ pub struct RwLockReadGuard<'a, T: ?Sized> {
///
/// [`write`]: method@RwLock::write
/// [`RwLock`]: struct@RwLock
-#[derive(Debug)]
pub struct RwLockWriteGuard<'a, T: ?Sized> {
- permit: ReleasingPermit<'a, T>,
- lock: &'a RwLock<T>,
+ s: &'a Semaphore,
+ data: *mut T,
+ marker: marker::PhantomData<&'a mut T>,
}
-// Wrapper arround Permit that releases on Drop
-#[derive(Debug)]
-struct ReleasingPermit<'a, T: ?Sized> {
- num_permits: u16,
- lock: &'a RwLock<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> ReleasingPermit<'a, T> {
- async fn acquire(
- lock: &'a RwLock<T>,
- num_permits: u16,
- ) -> Result<ReleasingPermit<'a, T>, AcquireError> {
- lock.s.acquire(num_permits.into()).await?;
- Ok(Self { num_permits, lock })
+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<T: ?Sized> Drop for ReleasingPermit<'_, T> {
+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.lock.s.release(self.num_permits as usize);
+ self.s.release(MAX_READS);
}
}
@@ -139,9 +435,11 @@ fn bounds() {
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>>();
@@ -155,8 +453,17 @@ fn bounds() {
// 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.
@@ -178,6 +485,27 @@ impl<T: ?Sized> RwLock<T> {
}
}
+ /// Creates a new instance of an `RwLock<T>` which is unlocked.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use tokio::sync::RwLock;
+ ///
+ /// static LOCK: RwLock<i32> = RwLock::const_new(5);
+ /// ```
+ #[cfg(all(feature = "parking_lot", not(all(loom, test))))]
+ #[cfg_attr(docsrs, doc(cfg(feature = "parking_lot")))]
+ pub const fn const_new(value: T) -> RwLock<T>
+ where
+ T: Sized,
+ {
+ RwLock {
+ c: UnsafeCell::new(value),
+ s: Semaphore::const_new(MAX_READS),
+ }
+ }
+
/// Locks this rwlock with shared read access, causing the current task
/// to yield until the lock has been acquired.
///
@@ -210,12 +538,16 @@ impl<T: ?Sized> RwLock<T> {
///}
/// ```
pub async fn read(&self) -> RwLockReadGuard<'_, T> {
- let permit = ReleasingPermit::acquire(self, 1).await.unwrap_or_else(|_| {
+ 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 { lock: self, permit }
+ RwLockReadGuard {
+ s: &self.s,
+ data: self.c.get(),
+ marker: marker::PhantomData,
+ }
}
/// Locks this rwlock with exclusive write access, causing the current task
@@ -241,15 +573,40 @@ impl<T: ?Sized> RwLock<T> {
///}
/// ```
pub async fn write(&self) -> RwLockWriteGuard<'_, T> {
- let permit = ReleasingPermit::acquire(self, MAX_READS as u16)
- .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 { lock: self, permit }
+ 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,
+ }
+ }
+
+ /// 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 get_mut(&mut self) -> &mut T {
+ unsafe {
+ // Safety: This is https://github.com/rust-lang/rust/pull/76936
+ &mut *self.c.get()
+ }
}
/// Consumes the lock, returning the underlying data.
@@ -265,7 +622,7 @@ impl<T: ?Sized> ops::Deref for RwLockReadGuard<'_, T> {
type Target = T;
fn deref(&self) -> &T {
- unsafe { &*self.lock.c.get() }
+ unsafe { &*self.data }
}
}
@@ -273,13 +630,13 @@ impl<T: ?Sized> ops::Deref for RwLockWriteGuard<'_, T> {
type Target = T;
fn deref(&self) -> &T {
- unsafe { &*self.lock.c.get() }
+ unsafe { &*self.data }
}
}
impl<T: ?Sized> ops::DerefMut for RwLockWriteGuard<'_, T> {
fn deref_mut(&mut self) -> &mut T {
- unsafe { &mut *self.lock.c.get() }
+ unsafe { &mut *self.data }
}
}
@@ -289,7 +646,7 @@ impl<T> From<T> for RwLock<T> {
}
}
-impl<T> Default for RwLock<T>
+impl<T: ?Sized> Default for RwLock<T>
where
T: Default,
{
generated by cgit v1.2.3 (git 2.25.1) at 2024-07-15 09:56:23 +0000