rustc_data_structures/sync/lock.rs
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//! This module implements a lock which only uses synchronization if `might_be_dyn_thread_safe` is true.
//! It implements `DynSend` and `DynSync` instead of the typical `Send` and `Sync` traits.
#![allow(dead_code)]
use std::fmt;
#[derive(Clone, Copy, PartialEq)]
pub enum Mode {
NoSync,
Sync,
}
use std::cell::{Cell, UnsafeCell};
use std::intrinsics::unlikely;
use std::marker::PhantomData;
use std::mem::ManuallyDrop;
use std::ops::{Deref, DerefMut};
use parking_lot::RawMutex;
use parking_lot::lock_api::RawMutex as _;
use crate::sync::{DynSend, DynSync, mode};
/// A guard holding mutable access to a `Lock` which is in a locked state.
#[must_use = "if unused the Lock will immediately unlock"]
pub struct LockGuard<'a, T> {
lock: &'a Lock<T>,
marker: PhantomData<&'a mut T>,
/// The synchronization mode of the lock. This is explicitly passed to let LLVM relate it
/// to the original lock operation.
mode: Mode,
}
impl<'a, T: 'a> Deref for LockGuard<'a, T> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
// SAFETY: We have shared access to the mutable access owned by this type,
// so we can give out a shared reference.
unsafe { &*self.lock.data.get() }
}
}
impl<'a, T: 'a> DerefMut for LockGuard<'a, T> {
#[inline]
fn deref_mut(&mut self) -> &mut T {
// SAFETY: We have mutable access to the data so we can give out a mutable reference.
unsafe { &mut *self.lock.data.get() }
}
}
impl<'a, T: 'a> Drop for LockGuard<'a, T> {
#[inline]
fn drop(&mut self) {
// SAFETY (union access): We get `self.mode` from the lock operation so it is consistent
// with the `lock.mode` state. This means we access the right union fields.
match self.mode {
Mode::NoSync => {
let cell = unsafe { &self.lock.mode_union.no_sync };
debug_assert!(cell.get());
cell.set(false);
}
// SAFETY (unlock): We know that the lock is locked as this type is a proof of that.
Mode::Sync => unsafe { self.lock.mode_union.sync.unlock() },
}
}
}
union ModeUnion {
/// Indicates if the cell is locked. Only used if `Lock.mode` is `NoSync`.
no_sync: ManuallyDrop<Cell<bool>>,
/// A lock implementation that's only used if `Lock.mode` is `Sync`.
sync: ManuallyDrop<RawMutex>,
}
/// The value representing a locked state for the `Cell`.
const LOCKED: bool = true;
/// A lock which only uses synchronization if `might_be_dyn_thread_safe` is true.
/// It implements `DynSend` and `DynSync` instead of the typical `Send` and `Sync`.
pub struct Lock<T> {
/// Indicates if synchronization is used via `mode_union.sync` if it's `Sync`, or if a
/// not thread safe cell is used via `mode_union.no_sync` if it's `NoSync`.
/// This is set on initialization and never changed.
mode: Mode,
mode_union: ModeUnion,
data: UnsafeCell<T>,
}
impl<T> Lock<T> {
#[inline(always)]
pub fn new(inner: T) -> Self {
let (mode, mode_union) = if unlikely(mode::might_be_dyn_thread_safe()) {
// Create the lock with synchronization enabled using the `RawMutex` type.
(Mode::Sync, ModeUnion { sync: ManuallyDrop::new(RawMutex::INIT) })
} else {
// Create the lock with synchronization disabled.
(Mode::NoSync, ModeUnion { no_sync: ManuallyDrop::new(Cell::new(!LOCKED)) })
};
Lock { mode, mode_union, data: UnsafeCell::new(inner) }
}
#[inline(always)]
pub fn into_inner(self) -> T {
self.data.into_inner()
}
#[inline(always)]
pub fn get_mut(&mut self) -> &mut T {
self.data.get_mut()
}
#[inline(always)]
pub fn try_lock(&self) -> Option<LockGuard<'_, T>> {
let mode = self.mode;
// SAFETY: This is safe since the union fields are used in accordance with `self.mode`.
match mode {
Mode::NoSync => {
let cell = unsafe { &self.mode_union.no_sync };
let was_unlocked = cell.get() != LOCKED;
if was_unlocked {
cell.set(LOCKED);
}
was_unlocked
}
Mode::Sync => unsafe { self.mode_union.sync.try_lock() },
}
.then(|| LockGuard { lock: self, marker: PhantomData, mode })
}
/// This acquires the lock assuming synchronization is in a specific mode.
///
/// Safety
/// This method must only be called with `Mode::Sync` if `might_be_dyn_thread_safe` was
/// true on lock creation.
#[inline(always)]
#[track_caller]
pub unsafe fn lock_assume(&self, mode: Mode) -> LockGuard<'_, T> {
#[inline(never)]
#[track_caller]
#[cold]
fn lock_held() -> ! {
panic!("lock was already held")
}
// SAFETY: This is safe since the union fields are used in accordance with `mode`
// which also must match `self.mode` due to the safety precondition.
unsafe {
match mode {
Mode::NoSync => {
if unlikely(self.mode_union.no_sync.replace(LOCKED) == LOCKED) {
lock_held()
}
}
Mode::Sync => self.mode_union.sync.lock(),
}
}
LockGuard { lock: self, marker: PhantomData, mode }
}
#[inline(always)]
#[track_caller]
pub fn lock(&self) -> LockGuard<'_, T> {
unsafe { self.lock_assume(self.mode) }
}
}
unsafe impl<T: DynSend> DynSend for Lock<T> {}
unsafe impl<T: DynSend> DynSync for Lock<T> {}
impl<T> Lock<T> {
#[inline(always)]
#[track_caller]
pub fn with_lock<F: FnOnce(&mut T) -> R, R>(&self, f: F) -> R {
f(&mut *self.lock())
}
#[inline(always)]
#[track_caller]
pub fn borrow(&self) -> LockGuard<'_, T> {
self.lock()
}
#[inline(always)]
#[track_caller]
pub fn borrow_mut(&self) -> LockGuard<'_, T> {
self.lock()
}
}
impl<T: Default> Default for Lock<T> {
#[inline]
fn default() -> Self {
Lock::new(T::default())
}
}
impl<T: fmt::Debug> fmt::Debug for Lock<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self.try_lock() {
Some(guard) => f.debug_struct("Lock").field("data", &&*guard).finish(),
None => {
struct LockedPlaceholder;
impl fmt::Debug for LockedPlaceholder {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.write_str("<locked>")
}
}
f.debug_struct("Lock").field("data", &LockedPlaceholder).finish()
}
}
}
}