Struct RwMutex

pub struct RwMutex<T: ?Sized> {
    lock: AtomicUsize,
    queue: WaitQueue,
    val: UnsafeCell<T>,
}

A mutex that provides data access to either one writer or many readers.

Overview

This mutex allows for multiple readers, or at most one writer to access at any point in time. The writer of this mutex has exclusive access to modify the underlying data, while the readers are allowed shared and read-only access.

The writing and reading portions cannot be active simultaneously, when one portion is in progress, the other portion will sleep. This is suitable for scenarios where the mutex is expected to be held for a period of time, which can avoid wasting CPU resources.

This implementation provides the upgradeable read mutex (upread mutex). The upread mutex can be upgraded to write mutex atomically, useful in scenarios where a decision to write is made after reading.

The type parameter T represents the data that this mutex is protecting. It is necessary for T to satisfy Send to be shared across tasks and Sync to permit concurrent access via readers. The Deref method (and DerefMut for the writer) is implemented for the RAII guards returned by the locking methods, which allows for the access to the protected data while the mutex is held.

Usage

The mutex can be used in scenarios where data needs to be read frequently but written to occasionally.

Use upread mutex in scenarios where related checking is performed before modification to effectively avoid deadlocks and improve efficiency.

Safety

Avoid using RwMutex in an interrupt context, as it may result in sleeping and never being awakened.

Examples

use ostd::sync::RwMutex;

let mutex = RwMutex::new(5)

// many read mutexes can be held at once
{
    let r1 = mutex.read();
    let r2 = mutex.read();
    assert_eq!(*r1, 5);
    assert_eq!(*r2, 5);
     
    // Upgradeable read mutex can share access to data with read mutexes
    let r3 = mutex.upread();
    assert_eq!(*r3, 5);
    drop(r1);
    drop(r2);
    // read mutexes are dropped at this point

    // An upread mutex can only be upgraded successfully after all the
    // read mutexes are released, otherwise it will spin-wait.
    let mut w1 = r3.upgrade();
    *w1 += 1;
    assert_eq!(*w1, 6);
}   // upread mutex are dropped at this point

{   
    // Only one write mutex can be held at a time
    let mut w2 = mutex.write();
    *w2 += 1;
    assert_eq!(*w2, 7);
}   // write mutex is dropped at this point

Fields

lock: AtomicUsize

The internal representation of the mutex state is as follows:

• Bit 63: Writer mutex.
• Bit 62: Upgradeable reader mutex.
• Bit 61: Indicates if an upgradeable reader is being upgraded.
• Bits 60-0: Reader mutex count.

queue: WaitQueue

Threads that fail to acquire the mutex will sleep on this waitqueue.

val: UnsafeCell<T>

Implementations

impl<T> RwMutex<T>

pub const fn new(val: T) -> Self

Creates a new read-write mutex with an initial value.

impl<T: ?Sized> RwMutex<T>

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

Acquires a read mutex and sleep until it can be acquired.

The calling thread will sleep until there are no writers or upgrading upreaders present. The implementation of WaitQueue guarantees the order in which other concurrent readers or writers waiting simultaneously will acquire the mutex.

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

Acquires a write mutex and sleep until it can be acquired.

The calling thread will sleep until there are no writers, upreaders, or readers present. The implementation of WaitQueue guarantees the order in which other concurrent readers or writers waiting simultaneously will acquire the mutex.

pub fn upread(&self) -> RwMutexUpgradeableGuard<'_, T>

Acquires a upread mutex and sleep until it can be acquired.

The calling thread will sleep until there are no writers or upreaders present. The implementation of WaitQueue guarantees the order in which other concurrent readers or writers waiting simultaneously will acquire the mutex.

Upreader will not block new readers until it tries to upgrade. Upreader and reader do not differ before invoking the upgread method. However, only one upreader can exist at any time to avoid deadlock in the upgread method.

pub fn try_read(&self) -> Option<RwMutexReadGuard<'_, T>>

Attempts to acquire a read mutex.

This function will never sleep and will return immediately.

pub fn try_write(&self) -> Option<RwMutexWriteGuard<'_, T>>

Attempts to acquire a write mutex.

This function will never sleep and will return immediately.

pub fn try_upread(&self) -> Option<RwMutexUpgradeableGuard<'_, T>>

Attempts to acquire a upread mutex.

This function will never sleep and will return immediately.

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

Returns a mutable reference to the underlying data.

This method is zero-cost: By holding a mutable reference to the lock, the compiler has already statically guaranteed that access to the data is exclusive.

Trait Implementations

impl<T: ?Sized + Debug> Debug for RwMutex<T>

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

Formats the value using the given formatter.

impl<T: ?Sized + Send> Send for RwMutex<T>

Because there can be more than one readers to get the T’s immutable ref, so T must be Sync to guarantee the sharing safety.

impl<T: ?Sized + Send + Sync> Sync for RwMutex<T>