[3.15] gh-157914: Revert "gh-135871: Reload lock internal state while spinning in PyMutex_LockTimed (gh-146064)" (gh-158082) (#158290)

Co-authored-by: Daniele Parmeggiani <daniele.parmeggiani@arm.com>

Author: miss-islington

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.

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;
         }