[3.8] gh-112275: Fix HEAD_LOCK deadlock in child process after fork (G…

…H-112336) (GH-123688) (#123713)

HEAD_LOCK is called from _PyEval_ReInitThreads->_PyThreadState_DeleteExcept before _PyRuntimeState_ReInitThreads reinit runtime->interpreters.mutex which might be locked before fork.

(cherry picked from commit 522799a)
(cherry picked from commit 0152431)

Co-authored-by: Łukasz Langa <lukasz@langa.pl>
Co-authored-by: ChuBoning <102216855+ChuBoning@users.noreply.github.com>

Misc/NEWS.d/next/Core_and_Builtins/2024-09-04-18-20-11.gh-issue-112275.W_iMiB.rst

@@ -0,0 +1,3 @@
+A deadlock involving ``pystate.c``'s ``HEAD_LOCK`` in ``posixmodule.c``
+at fork is now fixed. Patch by ChuBoning based on previous Python 3.12
+fix by Victor Stinner.

Modules/posixmodule.c

@@ -472,10 +472,10 @@ PyOS_AfterFork_Child(void)
 {
  _PyRuntimeState *runtime = &_PyRuntime;
  _PyGILState_Reinit(runtime);
+ _PyRuntimeState_ReInitThreads(runtime);
  _PyEval_ReInitThreads(runtime);
  _PyImport_ReInitLock();
  _PySignal_AfterFork();
- _PyRuntimeState_ReInitThreads(runtime);
  _PyInterpreterState_DeleteExceptMain(runtime);
 
  run_at_forkers(_PyInterpreterState_Get()->after_forkers_child, 0);