JuliaLang / julia / #37986

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Committed by web-flow

Commit Message

ReentrantLock: wakeup a single task on unlock and add a short spin (#56814)

I propose a change in the implementation of the `ReentrantLock` to
improve its overall throughput for short critical sections and fix the
quadratic wake-up behavior where each unlock schedules **all** waiting
tasks on the lock's wait queue.

This implementation follows the same principles of the `Mutex` in the
[parking_lot](https://github.com/Amanieu/parking_lot/tree/master) Rust
crate which is based on the Webkit
[WTF::ParkingLot](https://webkit.org/blog/6161/locking-in-webkit/)
class. Only the basic working principle is implemented here, further
improvements such as eventual fairness will be proposed separately.

The gist of the change is that we add one extra state to the lock,
essentially going from:
```
0x0 => The lock is not locked
0x1 => The lock is locked by exactly one task. No other task is waiting for it.
0x2 => The lock is locked and some other task tried to lock but failed (conflict)
```
To:
```
# PARKED_BIT | LOCKED_BIT | Description
#     0      |     0      | The lock is not locked, nor is anyone waiting for it.
# -----------+------------+------------------------------------------------------------------
#     0      |     1      | The lock is locked by exactly one task. No other task is
#            |            | waiting for it.
# -----------+------------+------------------------------------------------------------------
#     1      |     0      | The lock is not locked. One or more tasks are parked.
# -----------+------------+------------------------------------------------------------------
#     1      |     1      | The lock is locked by exactly one task. One or more tasks are
#            |            | parked waiting for the lock to become available.
#            |            | In this state, PARKED_BIT is only ever cleared when the cond_wait lock
#            |            | is held (i.e. on unlock). This ensures that
... (continued)

Run Details

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