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use std::cell::UnsafeCell;
use std::collections::HashMap;
use std::fmt;
use std::marker::PhantomData;
use std::mem;
use std::ops::{Deref, DerefMut};
use std::panic::{RefUnwindSafe, UnwindSafe};
use std::sync::{LockResult, PoisonError, TryLockError, TryLockResult};
use std::sync::{Mutex, RwLock, RwLockReadGuard, RwLockWriteGuard};
use std::thread::{self, ThreadId};
use crate::sync::once_lock::OnceLock;
use crate::CachePadded;
/// The number of shards per sharded lock. Must be a power of two.
const NUM_SHARDS: usize = 8;
/// A shard containing a single reader-writer lock.
struct Shard {
/// The inner reader-writer lock.
lock: RwLock<()>,
/// The write-guard keeping this shard locked.
///
/// Write operations will lock each shard and store the guard here. These guards get dropped at
/// the same time the big guard is dropped.
write_guard: UnsafeCell<Option<RwLockWriteGuard<'static, ()>>>,
}
/// A sharded reader-writer lock.
///
/// This lock is equivalent to [`RwLock`], except read operations are faster and write operations
/// are slower.
///
/// A `ShardedLock` is internally made of a list of *shards*, each being a [`RwLock`] occupying a
/// single cache line. Read operations will pick one of the shards depending on the current thread
/// and lock it. Write operations need to lock all shards in succession.
///
/// By splitting the lock into shards, concurrent read operations will in most cases choose
/// different shards and thus update different cache lines, which is good for scalability. However,
/// write operations need to do more work and are therefore slower than usual.
///
/// The priority policy of the lock is dependent on the underlying operating system's
/// implementation, and this type does not guarantee that any particular policy will be used.
///
/// # Poisoning
///
/// A `ShardedLock`, like [`RwLock`], will become poisoned on a panic. Note that it may only be
/// poisoned if a panic occurs while a write operation is in progress. If a panic occurs in any
/// read operation, the lock will not be poisoned.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::ShardedLock;
///
/// let lock = ShardedLock::new(5);
///
/// // Any number of read locks can be held at once.
/// {
/// let r1 = lock.read().unwrap();
/// let r2 = lock.read().unwrap();
/// assert_eq!(*r1, 5);
/// assert_eq!(*r2, 5);
/// } // Read locks are dropped at this point.
///
/// // However, only one write lock may be held.
/// {
/// let mut w = lock.write().unwrap();
/// *w += 1;
/// assert_eq!(*w, 6);
/// } // Write lock is dropped here.
/// ```
///
/// [`RwLock`]: std::sync::RwLock
pub struct ShardedLock<T: ?Sized> {
/// A list of locks protecting the internal data.
shards: Box<[CachePadded<Shard>]>,
/// The internal data.
value: UnsafeCell<T>,
}
unsafe impl<T: ?Sized + Send> Send for ShardedLock<T> {}
unsafe impl<T: ?Sized + Send + Sync> Sync for ShardedLock<T> {}
impl<T: ?Sized> UnwindSafe for ShardedLock<T> {}
impl<T: ?Sized> RefUnwindSafe for ShardedLock<T> {}
impl<T> ShardedLock<T> {
/// Creates a new sharded reader-writer lock.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::ShardedLock;
///
/// let lock = ShardedLock::new(5);
/// ```
pub fn new(value: T) -> ShardedLock<T> {
ShardedLock {
shards: (0..NUM_SHARDS)
.map(|_| {
CachePadded::new(Shard {
lock: RwLock::new(()),
write_guard: UnsafeCell::new(None),
})
})
.collect::<Box<[_]>>(),
value: UnsafeCell::new(value),
}
}
/// Consumes this lock, returning the underlying data.
///
/// # Errors
///
/// This method will return an error if the lock is poisoned. A lock gets poisoned when a write
/// operation panics.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::ShardedLock;
///
/// let lock = ShardedLock::new(String::new());
/// {
/// let mut s = lock.write().unwrap();
/// *s = "modified".to_owned();
/// }
/// assert_eq!(lock.into_inner().unwrap(), "modified");
/// ```
pub fn into_inner(self) -> LockResult<T> {
let is_poisoned = self.is_poisoned();
let inner = self.value.into_inner();
if is_poisoned {
Err(PoisonError::new(inner))
} else {
Ok(inner)
}
}
}
impl<T: ?Sized> ShardedLock<T> {
/// Returns `true` if the lock is poisoned.
///
/// If another thread can still access the lock, it may become poisoned at any time. A `false`
/// result should not be trusted without additional synchronization.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::ShardedLock;
/// use std::sync::Arc;
/// use std::thread;
///
/// let lock = Arc::new(ShardedLock::new(0));
/// let c_lock = lock.clone();
///
/// let _ = thread::spawn(move || {
/// let _lock = c_lock.write().unwrap();
/// panic!(); // the lock gets poisoned
/// }).join();
/// assert_eq!(lock.is_poisoned(), true);
/// ```
pub fn is_poisoned(&self) -> bool {
self.shards[0].lock.is_poisoned()
}
/// Returns a mutable reference to the underlying data.
///
/// Since this call borrows the lock mutably, no actual locking needs to take place.
///
/// # Errors
///
/// This method will return an error if the lock is poisoned. A lock gets poisoned when a write
/// operation panics.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::ShardedLock;
///
/// let mut lock = ShardedLock::new(0);
/// *lock.get_mut().unwrap() = 10;
/// assert_eq!(*lock.read().unwrap(), 10);
/// ```
pub fn get_mut(&mut self) -> LockResult<&mut T> {
let is_poisoned = self.is_poisoned();
let inner = unsafe { &mut *self.value.get() };
if is_poisoned {
Err(PoisonError::new(inner))
} else {
Ok(inner)
}
}
/// Attempts to acquire this lock with shared read access.
///
/// If the access could not be granted at this time, an error is returned. Otherwise, a guard
/// is returned which will release the shared access when it is dropped. This method does not
/// provide any guarantees with respect to the ordering of whether contentious readers or
/// writers will acquire the lock first.
///
/// # Errors
///
/// This method will return an error if the lock is poisoned. A lock gets poisoned when a write
/// operation panics.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::ShardedLock;
///
/// let lock = ShardedLock::new(1);
///
/// match lock.try_read() {
/// Ok(n) => assert_eq!(*n, 1),
/// Err(_) => unreachable!(),
/// };
/// ```
pub fn try_read(&self) -> TryLockResult<ShardedLockReadGuard<'_, T>> {
// Take the current thread index and map it to a shard index. Thread indices will tend to
// distribute shards among threads equally, thus reducing contention due to read-locking.
let current_index = current_index().unwrap_or(0);
let shard_index = current_index & (self.shards.len() - 1);
match self.shards[shard_index].lock.try_read() {
Ok(guard) => Ok(ShardedLockReadGuard {
lock: self,
_guard: guard,
_marker: PhantomData,
}),
Err(TryLockError::Poisoned(err)) => {
let guard = ShardedLockReadGuard {
lock: self,
_guard: err.into_inner(),
_marker: PhantomData,
};
Err(TryLockError::Poisoned(PoisonError::new(guard)))
}
Err(TryLockError::WouldBlock) => Err(TryLockError::WouldBlock),
}
}
/// Locks with shared read access, blocking the current thread until it can be acquired.
///
/// The calling thread will be blocked until there are no more writers which hold the lock.
/// There may be other readers currently inside the lock when this method returns. This method
/// does not provide any guarantees with respect to the ordering of whether contentious readers
/// or writers will acquire the lock first.
///
/// Returns a guard which will release the shared access when dropped.
///
/// # Errors
///
/// This method will return an error if the lock is poisoned. A lock gets poisoned when a write
/// operation panics.
///
/// # Panics
///
/// This method might panic when called if the lock is already held by the current thread.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::ShardedLock;
/// use std::sync::Arc;
/// use std::thread;
///
/// let lock = Arc::new(ShardedLock::new(1));
/// let c_lock = lock.clone();
///
/// let n = lock.read().unwrap();
/// assert_eq!(*n, 1);
///
/// thread::spawn(move || {
/// let r = c_lock.read();
/// assert!(r.is_ok());
/// }).join().unwrap();
/// ```
pub fn read(&self) -> LockResult<ShardedLockReadGuard<'_, T>> {
// Take the current thread index and map it to a shard index. Thread indices will tend to
// distribute shards among threads equally, thus reducing contention due to read-locking.
let current_index = current_index().unwrap_or(0);
let shard_index = current_index & (self.shards.len() - 1);
match self.shards[shard_index].lock.read() {
Ok(guard) => Ok(ShardedLockReadGuard {
lock: self,
_guard: guard,
_marker: PhantomData,
}),
Err(err) => Err(PoisonError::new(ShardedLockReadGuard {
lock: self,
_guard: err.into_inner(),
_marker: PhantomData,
})),
}
}
/// Attempts to acquire this lock with exclusive write access.
///
/// If the access could not be granted at this time, an error is returned. Otherwise, a guard
/// is returned which will release the exclusive access when it is dropped. This method does
/// not provide any guarantees with respect to the ordering of whether contentious readers or
/// writers will acquire the lock first.
///
/// # Errors
///
/// This method will return an error if the lock is poisoned. A lock gets poisoned when a write
/// operation panics.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::ShardedLock;
///
/// let lock = ShardedLock::new(1);
///
/// let n = lock.read().unwrap();
/// assert_eq!(*n, 1);
///
/// assert!(lock.try_write().is_err());
/// ```
pub fn try_write(&self) -> TryLockResult<ShardedLockWriteGuard<'_, T>> {
let mut poisoned = false;
let mut blocked = None;
// Write-lock each shard in succession.
for (i, shard) in self.shards.iter().enumerate() {
let guard = match shard.lock.try_write() {
Ok(guard) => guard,
Err(TryLockError::Poisoned(err)) => {
poisoned = true;
err.into_inner()
}
Err(TryLockError::WouldBlock) => {
blocked = Some(i);
break;
}
};
// Store the guard into the shard.
unsafe {
let guard: RwLockWriteGuard<'static, ()> = mem::transmute(guard);
let dest: *mut _ = shard.write_guard.get();
*dest = Some(guard);
}
}
if let Some(i) = blocked {
// Unlock the shards in reverse order of locking.
for shard in self.shards[0..i].iter().rev() {
unsafe {
let dest: *mut _ = shard.write_guard.get();
let guard = mem::replace(&mut *dest, None);
drop(guard);
}
}
Err(TryLockError::WouldBlock)
} else if poisoned {
let guard = ShardedLockWriteGuard {
lock: self,
_marker: PhantomData,
};
Err(TryLockError::Poisoned(PoisonError::new(guard)))
} else {
Ok(ShardedLockWriteGuard {
lock: self,
_marker: PhantomData,
})
}
}
/// Locks with exclusive write access, blocking the current thread until it can be acquired.
///
/// The calling thread will be blocked until there are no more writers which hold the lock.
/// There may be other readers currently inside the lock when this method returns. This method
/// does not provide any guarantees with respect to the ordering of whether contentious readers
/// or writers will acquire the lock first.
///
/// Returns a guard which will release the exclusive access when dropped.
///
/// # Errors
///
/// This method will return an error if the lock is poisoned. A lock gets poisoned when a write
/// operation panics.
///
/// # Panics
///
/// This method might panic when called if the lock is already held by the current thread.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::ShardedLock;
///
/// let lock = ShardedLock::new(1);
///
/// let mut n = lock.write().unwrap();
/// *n = 2;
///
/// assert!(lock.try_read().is_err());
/// ```
pub fn write(&self) -> LockResult<ShardedLockWriteGuard<'_, T>> {
let mut poisoned = false;
// Write-lock each shard in succession.
for shard in self.shards.iter() {
let guard = match shard.lock.write() {
Ok(guard) => guard,
Err(err) => {
poisoned = true;
err.into_inner()
}
};
// Store the guard into the shard.
unsafe {
let guard: RwLockWriteGuard<'_, ()> = guard;
let guard: RwLockWriteGuard<'static, ()> = mem::transmute(guard);
let dest: *mut _ = shard.write_guard.get();
*dest = Some(guard);
}
}
if poisoned {
Err(PoisonError::new(ShardedLockWriteGuard {
lock: self,
_marker: PhantomData,
}))
} else {
Ok(ShardedLockWriteGuard {
lock: self,
_marker: PhantomData,
})
}
}
}
impl<T: ?Sized + fmt::Debug> fmt::Debug for ShardedLock<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self.try_read() {
Ok(guard) => f
.debug_struct("ShardedLock")
.field("data", &&*guard)
.finish(),
Err(TryLockError::Poisoned(err)) => f
.debug_struct("ShardedLock")
.field("data", &&**err.get_ref())
.finish(),
Err(TryLockError::WouldBlock) => {
struct LockedPlaceholder;
impl fmt::Debug for LockedPlaceholder {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.write_str("<locked>")
}
}
f.debug_struct("ShardedLock")
.field("data", &LockedPlaceholder)
.finish()
}
}
}
}
impl<T: Default> Default for ShardedLock<T> {
fn default() -> ShardedLock<T> {
ShardedLock::new(Default::default())
}
}
impl<T> From<T> for ShardedLock<T> {
fn from(t: T) -> Self {
ShardedLock::new(t)
}
}
/// A guard used to release the shared read access of a [`ShardedLock`] when dropped.
pub struct ShardedLockReadGuard<'a, T: ?Sized> {
lock: &'a ShardedLock<T>,
_guard: RwLockReadGuard<'a, ()>,
_marker: PhantomData<RwLockReadGuard<'a, T>>,
}
unsafe impl<T: ?Sized + Sync> Sync for ShardedLockReadGuard<'_, T> {}
impl<T: ?Sized> Deref for ShardedLockReadGuard<'_, T> {
type Target = T;
fn deref(&self) -> &T {
unsafe { &*self.lock.value.get() }
}
}
impl<T: fmt::Debug> fmt::Debug for ShardedLockReadGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("ShardedLockReadGuard")
.field("lock", &self.lock)
.finish()
}
}
impl<T: ?Sized + fmt::Display> fmt::Display for ShardedLockReadGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(**self).fmt(f)
}
}
/// A guard used to release the exclusive write access of a [`ShardedLock`] when dropped.
pub struct ShardedLockWriteGuard<'a, T: ?Sized> {
lock: &'a ShardedLock<T>,
_marker: PhantomData<RwLockWriteGuard<'a, T>>,
}
unsafe impl<T: ?Sized + Sync> Sync for ShardedLockWriteGuard<'_, T> {}
impl<T: ?Sized> Drop for ShardedLockWriteGuard<'_, T> {
fn drop(&mut self) {
// Unlock the shards in reverse order of locking.
for shard in self.lock.shards.iter().rev() {
unsafe {
let dest: *mut _ = shard.write_guard.get();
let guard = mem::replace(&mut *dest, None);
drop(guard);
}
}
}
}
impl<T: fmt::Debug> fmt::Debug for ShardedLockWriteGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("ShardedLockWriteGuard")
.field("lock", &self.lock)
.finish()
}
}
impl<T: ?Sized + fmt::Display> fmt::Display for ShardedLockWriteGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(**self).fmt(f)
}
}
impl<T: ?Sized> Deref for ShardedLockWriteGuard<'_, T> {
type Target = T;
fn deref(&self) -> &T {
unsafe { &*self.lock.value.get() }
}
}
impl<T: ?Sized> DerefMut for ShardedLockWriteGuard<'_, T> {
fn deref_mut(&mut self) -> &mut T {
unsafe { &mut *self.lock.value.get() }
}
}
/// Returns a `usize` that identifies the current thread.
///
/// Each thread is associated with an 'index'. While there are no particular guarantees, indices
/// usually tend to be consecutive numbers between 0 and the number of running threads.
///
/// Since this function accesses TLS, `None` might be returned if the current thread's TLS is
/// tearing down.
#[inline]
fn current_index() -> Option<usize> {
REGISTRATION.try_with(|reg| reg.index).ok()
}
/// The global registry keeping track of registered threads and indices.
struct ThreadIndices {
/// Mapping from `ThreadId` to thread index.
mapping: HashMap<ThreadId, usize>,
/// A list of free indices.
free_list: Vec<usize>,
/// The next index to allocate if the free list is empty.
next_index: usize,
}
fn thread_indices() -> &'static Mutex<ThreadIndices> {
static THREAD_INDICES: OnceLock<Mutex<ThreadIndices>> = OnceLock::new();
fn init() -> Mutex<ThreadIndices> {
Mutex::new(ThreadIndices {
mapping: HashMap::new(),
free_list: Vec::new(),
next_index: 0,
})
}
THREAD_INDICES.get_or_init(init)
}
/// A registration of a thread with an index.
///
/// When dropped, unregisters the thread and frees the reserved index.
struct Registration {
index: usize,
thread_id: ThreadId,
}
impl Drop for Registration {
fn drop(&mut self) {
let mut indices = thread_indices().lock().unwrap();
indices.mapping.remove(&self.thread_id);
indices.free_list.push(self.index);
}
}
thread_local! {
static REGISTRATION: Registration = {
let thread_id = thread::current().id();
let mut indices = thread_indices().lock().unwrap();
let index = match indices.free_list.pop() {
Some(i) => i,
None => {
let i = indices.next_index;
indices.next_index += 1;
i
}
};
indices.mapping.insert(thread_id, index);
Registration {
index,
thread_id,
}
};
}