From 7217ad07daa0a404fcf4452d8cf44fabb2b7c1de Mon Sep 17 00:00:00 2001
From: Daniele Parmeggiani <daniele.parmeggiani@arm.com>
Date: Fri, 27 Feb 2026 16:21:11 +0000
Subject: [PATCH 1/2] gh-135871: Reload lock internal state while spinning in
 `_PyMutex_LockTimed`

---
 Python/lock.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/Python/lock.c b/Python/lock.c
index ad97bfd93c8495..03bdc21b70f230 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_COUNT = 3;
 #else
 static const int MAX_SPIN_COUNT = 0;
+static const int RELOAD_SPIN_COUNT = 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_COUNT) == 0) {
+                v = _Py_atomic_load_uint8_relaxed(&m->_bits);
+            }
             continue;
         }
 

From 54932d638bf3bbe0cac46b0acc1523f64d844307 Mon Sep 17 00:00:00 2001
From: "blurb-it[bot]" <43283697+blurb-it[bot]@users.noreply.github.com>
Date: Thu, 19 Mar 2026 16:16:44 +0000
Subject: [PATCH 2/2] =?UTF-8?q?=F0=9F=93=9C=F0=9F=A4=96=20Added=20by=20blu?=
 =?UTF-8?q?rb=5Fit.?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 .../2026-03-19-16-16-40.gh-issue-135871.jSExZ3.rst               | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2026-03-19-16-16-40.gh-issue-135871.jSExZ3.rst

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.