https://github.com/python/cpython/commit/daa159f98b3689ae6a587bfb978b931762f9dbc9
commit: daa159f98b3689ae6a587bfb978b931762f9dbc9
branch: main
author: Daniele Parmeggiani <[email protected]>
committer: colesbury <[email protected]>
date: 2026-03-23T16:55:06-04:00
summary:
gh-135871: Reload lock internal state while spinning in `PyMutex_LockTimed`
(gh-146064)
Add atomic loads in the slow path of PyMutex to increase the number
of lock acquisitions per second that threads can make on a shared mutex.
files:
A
Misc/NEWS.d/next/Core_and_Builtins/2026-03-19-16-16-40.gh-issue-135871.jSExZ3.rst
M Python/lock.c
diff --git
a/Misc/NEWS.d/next/Core_and_Builtins/2026-03-19-16-16-40.gh-issue-135871.jSExZ3.rst
b/Misc/NEWS.d/next/Core_and_Builtins/2026-03-19-16-16-40.gh-issue-135871.jSExZ3.rst
new file mode 100644
index 00000000000000..29103e46906487
--- /dev/null
+++
b/Misc/NEWS.d/next/Core_and_Builtins/2026-03-19-16-16-40.gh-issue-135871.jSExZ3.rst
@@ -0,0 +1 @@
+Improve multithreaded scaling of PyMutex in low-contention scenarios by
reloading the lock's internal state, without slowing down high-contention
scenarios.
diff --git a/Python/lock.c b/Python/lock.c
index ad97bfd93c8495..752a5899e088a5 100644
--- a/Python/lock.c
+++ b/Python/lock.c
@@ -27,8 +27,10 @@ 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 {
@@ -79,6 +81,16 @@ _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.
@@ -92,6 +104,9 @@ _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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