From: Peter Hawkins <phawkins@google.com>
Date: Fri, 31 Jul 2026 01:28:20 +0000 (-0700)
Subject: gh-152075: Avoid lock contention in _Py_Specialize_LoadGlobal under free threading... 
X-Git-Url: http://git.ipfire.org/gitweb/?a=commitdiff_plain;h=ac8ba0ca5a04cd7b27b44822d31df640e817fd78;p=thirdparty%2FPython%2Fcpython.git

gh-152075: Avoid lock contention in _Py_Specialize_LoadGlobal under free threading (gh-153720)

Under high thread concurrency in free-threaded builds, `_Py_Specialize_LoadGlobal` suffers from lock contention when acquiring the critical section mutexes for the `globals` and `builtins` dictionaries during bytecode specialization.

This PR skips LOAD_GLOBAL bytecode specialization if acquiring the two object mutexes would block.
---

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-07-14-20-19-48.gh-issue-152075.V9Rwhp.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-07-14-20-19-48.gh-issue-152075.V9Rwhp.rst
new file mode 100644
index 000000000000..84c7d36b97ac
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-07-14-20-19-48.gh-issue-152075.V9Rwhp.rst
@@ -0,0 +1,2 @@
+Reduced lock contention during LOAD_GLOBAL bytecode specialization under
+free threading.
diff --git a/Python/specialize.c b/Python/specialize.c
index 773b55c329c7..05cb76ff015f 100644
--- a/Python/specialize.c
+++ b/Python/specialize.c
@@ -1457,6 +1457,14 @@ _Py_Specialize_LoadGlobal(
     PyObject *globals, PyObject *builtins,
     _Py_CODEUNIT *instr, PyObject *name)
 {
+#ifdef Py_GIL_DISABLED
+    if (PyMutex_IsLocked(&globals->ob_mutex) || PyMutex_IsLocked(&builtins->ob_mutex)) {
+        // Skip specialization if either dictionary is locked to avoid lock
+        // contention.
+        unspecialize(instr);
+        return;
+    }
+#endif
     Py_BEGIN_CRITICAL_SECTION2(globals, builtins);
     specialize_load_global_lock_held(globals, builtins, instr, name);
     Py_END_CRITICAL_SECTION2();