third_party/rust_crates/vendor/parking_lot-0.9.0/src/raw_mutex.rs

// Copyright 2016 Amanieu d'Antras
//
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
// http://opensource.org/licenses/MIT>, at your option. This file may not be
// copied, modified, or distributed except according to those terms.

use crate::{deadlock, util};
#[cfg(has_sized_atomics)]
use core::sync::atomic::AtomicU8;
#[cfg(not(has_sized_atomics))]
use core::sync::atomic::AtomicUsize as AtomicU8;
use core::{sync::atomic::Ordering, time::Duration};
use lock_api::{GuardNoSend, RawMutex as RawMutexTrait, RawMutexFair, RawMutexTimed};
use parking_lot_core::{self, ParkResult, SpinWait, UnparkResult, UnparkToken, DEFAULT_PARK_TOKEN};
use std::time::Instant;

#[cfg(has_sized_atomics)]
type U8 = u8;
#[cfg(not(has_sized_atomics))]
type U8 = usize;

// UnparkToken used to indicate that that the target thread should attempt to
// lock the mutex again as soon as it is unparked.
pub(crate) const TOKEN_NORMAL: UnparkToken = UnparkToken(0);

// UnparkToken used to indicate that the mutex is being handed off to the target
// thread directly without unlocking it.
pub(crate) const TOKEN_HANDOFF: UnparkToken = UnparkToken(1);

const LOCKED_BIT: U8 = 1;
const PARKED_BIT: U8 = 2;

/// Raw mutex type backed by the parking lot.
pub struct RawMutex {
    state: AtomicU8,
}

unsafe impl RawMutexTrait for RawMutex {
    const INIT: RawMutex = RawMutex { state: AtomicU8::new(0) };

    type GuardMarker = GuardNoSend;

    #[inline]
    fn lock(&self) {
        if self
            .state
            .compare_exchange_weak(0, LOCKED_BIT, Ordering::Acquire, Ordering::Relaxed)
            .is_err()
        {
            self.lock_slow(None);
        }
        unsafe { deadlock::acquire_resource(self as *const _ as usize) };
    }

    #[inline]
    fn try_lock(&self) -> bool {
        let mut state = self.state.load(Ordering::Relaxed);
        loop {
            if state & LOCKED_BIT != 0 {
                return false;
            }
            match self.state.compare_exchange_weak(
                state,
                state | LOCKED_BIT,
                Ordering::Acquire,
                Ordering::Relaxed,
            ) {
                Ok(_) => {
                    unsafe { deadlock::acquire_resource(self as *const _ as usize) };
                    return true;
                }
                Err(x) => state = x,
            }
        }
    }

    #[inline]
    fn unlock(&self) {
        unsafe { deadlock::release_resource(self as *const _ as usize) };
        if self.state.compare_exchange(LOCKED_BIT, 0, Ordering::Release, Ordering::Relaxed).is_ok()
        {
            return;
        }
        self.unlock_slow(false);
    }
}

unsafe impl RawMutexFair for RawMutex {
    #[inline]
    fn unlock_fair(&self) {
        unsafe { deadlock::release_resource(self as *const _ as usize) };
        if self.state.compare_exchange(LOCKED_BIT, 0, Ordering::Release, Ordering::Relaxed).is_ok()
        {
            return;
        }
        self.unlock_slow(true);
    }

    #[inline]
    fn bump(&self) {
        if self.state.load(Ordering::Relaxed) & PARKED_BIT != 0 {
            self.bump_slow();
        }
    }
}

unsafe impl RawMutexTimed for RawMutex {
    type Duration = Duration;
    type Instant = Instant;

    #[inline]
    fn try_lock_until(&self, timeout: Instant) -> bool {
        let result = if self
            .state
            .compare_exchange_weak(0, LOCKED_BIT, Ordering::Acquire, Ordering::Relaxed)
            .is_ok()
        {
            true
        } else {
            self.lock_slow(Some(timeout))
        };
        if result {
            unsafe { deadlock::acquire_resource(self as *const _ as usize) };
        }
        result
    }

    #[inline]
    fn try_lock_for(&self, timeout: Duration) -> bool {
        let result = if self
            .state
            .compare_exchange_weak(0, LOCKED_BIT, Ordering::Acquire, Ordering::Relaxed)
            .is_ok()
        {
            true
        } else {
            self.lock_slow(util::to_deadline(timeout))
        };
        if result {
            unsafe { deadlock::acquire_resource(self as *const _ as usize) };
        }
        result
    }
}

impl RawMutex {
    // Used by Condvar when requeuing threads to us, must be called while
    // holding the queue lock.
    #[inline]
    pub(crate) fn mark_parked_if_locked(&self) -> bool {
        let mut state = self.state.load(Ordering::Relaxed);
        loop {
            if state & LOCKED_BIT == 0 {
                return false;
            }
            match self.state.compare_exchange_weak(
                state,
                state | PARKED_BIT,
                Ordering::Relaxed,
                Ordering::Relaxed,
            ) {
                Ok(_) => return true,
                Err(x) => state = x,
            }
        }
    }

    // Used by Condvar when requeuing threads to us, must be called while
    // holding the queue lock.
    #[inline]
    pub(crate) fn mark_parked(&self) {
        self.state.fetch_or(PARKED_BIT, Ordering::Relaxed);
    }

    #[cold]
    fn lock_slow(&self, timeout: Option<Instant>) -> bool {
        let mut spinwait = SpinWait::new();
        let mut state = self.state.load(Ordering::Relaxed);
        loop {
            // Grab the lock if it isn't locked, even if there is a queue on it
            if state & LOCKED_BIT == 0 {
                match self.state.compare_exchange_weak(
                    state,
                    state | LOCKED_BIT,
                    Ordering::Acquire,
                    Ordering::Relaxed,
                ) {
                    Ok(_) => return true,
                    Err(x) => state = x,
                }
                continue;
            }

            // If there is no queue, try spinning a few times
            if state & PARKED_BIT == 0 && spinwait.spin() {
                state = self.state.load(Ordering::Relaxed);
                continue;
            }

            // Set the parked bit
            if state & PARKED_BIT == 0 {
                if let Err(x) = self.state.compare_exchange_weak(
                    state,
                    state | PARKED_BIT,
                    Ordering::Relaxed,
                    Ordering::Relaxed,
                ) {
                    state = x;
                    continue;
                }
            }

            // Park our thread until we are woken up by an unlock
            unsafe {
                let addr = self as *const _ as usize;
                let validate = || self.state.load(Ordering::Relaxed) == LOCKED_BIT | PARKED_BIT;
                let before_sleep = || {};
                let timed_out = |_, was_last_thread| {
                    // Clear the parked bit if we were the last parked thread
                    if was_last_thread {
                        self.state.fetch_and(!PARKED_BIT, Ordering::Relaxed);
                    }
                };
                match parking_lot_core::park(
                    addr,
                    validate,
                    before_sleep,
                    timed_out,
                    DEFAULT_PARK_TOKEN,
                    timeout,
                ) {
                    // The thread that unparked us passed the lock on to us
                    // directly without unlocking it.
                    ParkResult::Unparked(TOKEN_HANDOFF) => return true,

                    // We were unparked normally, try acquiring the lock again
                    ParkResult::Unparked(_) => (),

                    // The validation function failed, try locking again
                    ParkResult::Invalid => (),

                    // Timeout expired
                    ParkResult::TimedOut => return false,
                }
            }

            // Loop back and try locking again
            spinwait.reset();
            state = self.state.load(Ordering::Relaxed);
        }
    }

    #[cold]
    fn unlock_slow(&self, force_fair: bool) {
        // Unpark one thread and leave the parked bit set if there might
        // still be parked threads on this address.
        unsafe {
            let addr = self as *const _ as usize;
            let callback = |result: UnparkResult| {
                // If we are using a fair unlock then we should keep the
                // mutex locked and hand it off to the unparked thread.
                if result.unparked_threads != 0 && (force_fair || result.be_fair) {
                    // Clear the parked bit if there are no more parked
                    // threads.
                    if !result.have_more_threads {
                        self.state.store(LOCKED_BIT, Ordering::Relaxed);
                    }
                    return TOKEN_HANDOFF;
                }

                // Clear the locked bit, and the parked bit as well if there
                // are no more parked threads.
                if result.have_more_threads {
                    self.state.store(PARKED_BIT, Ordering::Release);
                } else {
                    self.state.store(0, Ordering::Release);
                }
                TOKEN_NORMAL
            };
            parking_lot_core::unpark_one(addr, callback);
        }
    }

    #[cold]
    fn bump_slow(&self) {
        unsafe { deadlock::release_resource(self as *const _ as usize) };
        self.unlock_slow(true);
        self.lock();
    }
}