gh-108724: Add PyMutex and _PyParkingLot APIs by colesbury · Pull Request #109344 · python/cpython

colesbury

`PyMutex` is a one byte lock with fast, inlineable lock and unlock
functions for the common uncontended case. The design is based on
WebKit's `WTF::Lock`.

PyMutex is built using the `_PyParkingLot` APIs, which provides a
cross-platform futex-like API (based on WebKit's `WTF::ParkingLot`).
This internal API will be used for building other synchronization
primitives used to implement PEP 703, such as one-time initialization
and events.

This was referenced

Sep 13, 2023

The lock tests were being picked up twice: once in test_lock.py and once
in Test_testinternalcapi from test_misc.py. The Test_testinternalcapi
was not skipping tests when the platform doesn't have threads. This
moves the tests to test_misc.py, doesn't include them in
Test_testinternalcapi, and skips them if the platform doesn't support
threads.

csm10495 pushed a commit to csm10495/cpython that referenced this pull request

Sep 28, 2023

PyMutex is a one byte lock with fast, inlineable lock and unlock functions for the common uncontended case. The design is based on WebKit's WTF::Lock.

PyMutex is built using the _PyParkingLot APIs, which provides a cross-platform futex-like API (based on WebKit's WTF::ParkingLot). This internal API will be used for building other synchronization primitives used to implement PEP 703, such as one-time initialization and events.

This also includes tests and a mini benchmark in Tools/lockbench/lockbench.py to compare with the existing PyThread_type_lock.

Uncontended acquisition + release:
* Linux (x86-64): PyMutex: 11 ns, PyThread_type_lock: 44 ns
* macOS (arm64): PyMutex: 13 ns, PyThread_type_lock: 18 ns
* Windows (x86-64): PyMutex: 13 ns, PyThread_type_lock: 38 ns

PR Overview:

The primary purpose of this PR is to implement PyMutex, but there are a number of support pieces (described below).

* PyMutex: A 1-byte lock that doesn't require memory allocation to initialize and is generally faster than the existing PyThread_type_lock. The API is internal only for now.
* _PyParking_Lot: A futex-like API based on the API of the same name in WebKit. Used to implement PyMutex.
* _PyRawMutex: A word sized lock used to implement _PyParking_Lot.
* PyEvent: A one time event. This was used a bunch in the "nogil" fork and is useful for testing the PyMutex implementation, so I've included it as part of the PR.
* pycore_llist.h: Defines common operations on doubly-linked list. Not strictly necessary (could do the list operations manually), but they come up frequently in the "nogil" fork. ( Similar to https://man.freebsd.org/cgi/man.cgi?queue)

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Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>

This was referenced

Oct 13, 2023