https://github.com/python/cpython/commit/baec76438e32f50983f48c1658a55015b036204b
commit: baec76438e32f50983f48c1658a55015b036204b
branch: main
author: Daniele Parmeggiani <[email protected]>
committer: corona10 <[email protected]>
date: 2026-09-27T17:29:07Z
summary:
gh-157914: Revert "gh-135871: Reload lock internal state while spinning in
`PyMutex_LockTimed` (gh-146064)" (gh-158082)
files:
A
Misc/NEWS.d/next/Core_and_Builtins/2026-09-24-17-30-48.gh-issue-157914.5fVJob.rst
M Python/lock.c
diff --git
a/Misc/NEWS.d/next/Core_and_Builtins/2026-09-24-17-30-48.gh-issue-157914.5fVJob.rst
b/Misc/NEWS.d/next/Core_and_Builtins/2026-09-24-17-30-48.gh-issue-157914.5fVJob.rst
new file mode 100644
index 00000000000000..b381a568cf14ac
--- /dev/null
+++
b/Misc/NEWS.d/next/Core_and_Builtins/2026-09-24-17-30-48.gh-issue-157914.5fVJob.rst
@@ -0,0 +1,2 @@
+Revert :gh:`146064`. The linked PyMutex change was
+causing a significant performance regression in some cases.
diff --git a/Python/lock.c b/Python/lock.c
index b636b91e79678c..b669e3dcdbc999 100644
--- a/Python/lock.c
+++ b/Python/lock.c
@@ -27,10 +27,8 @@ static const PyTime_t TIME_TO_BE_FAIR_NS = 1000*1000;
// enabled.
#if Py_GIL_DISABLED
static const int MAX_SPIN_COUNT = 40;
-static const int RELOAD_SPIN_MASK = 3;
#else
static const int MAX_SPIN_COUNT = 0;
-static const int RELOAD_SPIN_MASK = 1;
#endif
struct mutex_entry {
@@ -81,16 +79,6 @@ _PyMutex_LockTimed(PyMutex *m, PyTime_t timeout,
_PyLockFlags flags)
};
Py_ssize_t spin_count = 0;
-#ifdef Py_GIL_DISABLED
- // Using thread-id as a way of reducing contention further in the reload
below.
- // It adds a pseudo-random starting offset to the recurrence, so that
threads
- // are less likely to try and run compare-exchange at the same time.
- // The lower bits of platform thread ids are likely to not be random,
- // hence the right shift.
- const Py_ssize_t tid = (Py_ssize_t)(_Py_ThreadId() >> 12);
-#else
- const Py_ssize_t tid = 0;
-#endif
for (;;) {
if ((v & _Py_LOCKED) == 0) {
// The lock is unlocked. Try to grab it.
@@ -104,9 +92,6 @@ _PyMutex_LockTimed(PyMutex *m, PyTime_t timeout,
_PyLockFlags flags)
// Spin for a bit.
_Py_yield();
spin_count++;
- if (((spin_count + tid) & RELOAD_SPIN_MASK) == 0) {
- v = _Py_atomic_load_uint8_relaxed(&m->_bits);
- }
continue;
}
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