rust/compiler/rustc_data_structures/src/sync/lock.rs

//! 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.
//!
//! When `cfg(parallel_compiler)` is not set, the lock is instead a wrapper around `RefCell`.

#![allow(dead_code)]

use std::fmt;

#[cfg(parallel_compiler)]
pub use maybe_sync::*;
#[cfg(not(parallel_compiler))]
pub use no_sync::*;

#[derive(Clone, Copy, PartialEq)]
pub enum Mode {
    NoSync,
    Sync,
}

mod maybe_sync {
    use super::Mode;
    use crate::sync::mode;
    #[cfg(parallel_compiler)]
    use crate::sync::{DynSend, DynSync};
    use parking_lot::lock_api::RawMutex as _;
    use parking_lot::RawMutex;
    use std::cell::Cell;
    use std::cell::UnsafeCell;
    use std::intrinsics::unlikely;
    use std::marker::PhantomData;
    use std::mem::ManuallyDrop;
    use std::ops::{Deref, DerefMut};

    /// 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_eq!(cell.get(), true);
                    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) }
        }
    }

    #[cfg(parallel_compiler)]
    unsafe impl<T: DynSend> DynSend for Lock<T> {}
    #[cfg(parallel_compiler)]
    unsafe impl<T: DynSend> DynSync for Lock<T> {}
}

mod no_sync {
    use super::Mode;
    use std::cell::RefCell;

    pub use std::cell::RefMut as LockGuard;

    pub struct Lock<T>(RefCell<T>);

    impl<T> Lock<T> {
        #[inline(always)]
        pub fn new(inner: T) -> Self {
            Lock(RefCell::new(inner))
        }

        #[inline(always)]
        pub fn into_inner(self) -> T {
            self.0.into_inner()
        }

        #[inline(always)]
        pub fn get_mut(&mut self) -> &mut T {
            self.0.get_mut()
        }

        #[inline(always)]
        pub fn try_lock(&self) -> Option<LockGuard<'_, T>> {
            self.0.try_borrow_mut().ok()
        }

        #[inline(always)]
        #[track_caller]
        // This is unsafe to match the API for the `parallel_compiler` case.
        pub unsafe fn lock_assume(&self, _mode: Mode) -> LockGuard<'_, T> {
            self.0.borrow_mut()
        }

        #[inline(always)]
        #[track_caller]
        pub fn lock(&self) -> LockGuard<'_, T> {
            self.0.borrow_mut()
        }
    }
}

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()
            }
        }
    }
}