gh-108724: Add PyMutex and _PyParkingLot APIs (gh-109344)

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)

---------

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

1 files changed, 5 insertions, 0 deletions

diff --git a/Python/pystate.c b/Python/pystate.c
index 08cf6f0..71ff3e5 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -9,6 +9,7 @@
 #include "pycore_frame.h"
 #include "pycore_initconfig.h"    // _PyStatus_OK()
 #include "pycore_object.h"        // _PyType_InitCache()
+#include "pycore_parking_lot.h"   // _PyParkingLot_AfterFork()
 #include "pycore_pyerrors.h"      // _PyErr_Clear()
 #include "pycore_pylifecycle.h"   // _PyAST_Fini()
 #include "pycore_pymem.h"         // _PyMem_SetDefaultAllocator()
@@ -554,6 +555,10 @@ _PyRuntimeState_ReInitThreads(_PyRuntimeState *runtime)
 
     PyMem_SetAllocator(PYMEM_DOMAIN_RAW, &old_alloc);
 
+    // Clears the parking lot. Any waiting threads are dead. This must be
+    // called before releasing any locks that use the parking lot.
+    _PyParkingLot_AfterFork();
+
     /* bpo-42540: id_mutex is freed by _PyInterpreterState_Delete, which does
      * not force the default allocator. */
     reinit_err += _PyThread_at_fork_reinit(&runtime->interpreters.main->id_mutex);