From: Sam Gross <colesbury@gmail.com>
Date: Thu, 27 Feb 2025 18:57:19 +0000 (-0500)
Subject: gh-130091: Reorder `_PyThreadState_Attach` to avoid data race (gh-130092)
X-Git-Tag: v3.14.0a6~233
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=7aeaa5af2c32f1f87b59f36f89e19284e24b26b9;p=thirdparty%2FPython%2Fcpython.git

gh-130091: Reorder `_PyThreadState_Attach` to avoid data race (gh-130092)

This moves `tstate_activate()` down to avoid a data race in the free
threading build on the `_PyRuntime`'s thread-local `autoTSSkey`. This
key is deleted during runtime finalization, which may happen
concurrently with a call to `_PyThreadState_Attach`.

The earlier `tstate_try/wait_attach` ensures that the thread is blocked
before it attempts to access the deleted `autoTSSkey`.

This fixes a TSAN reported data race in
`test_threading.test_import_from_another_thread`.
---

diff --git a/Python/pystate.c b/Python/pystate.c
index 9ebd9fdea607..72e2627e95b5 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -2099,11 +2099,10 @@ _PyThreadState_Attach(PyThreadState *tstate)
 
         // XXX assert(tstate_is_alive(tstate));
         current_fast_set(&_PyRuntime, tstate);
-        tstate_activate(tstate);
-
         if (!tstate_try_attach(tstate)) {
             tstate_wait_attach(tstate);
         }
+        tstate_activate(tstate);
 
 #ifdef Py_GIL_DISABLED
         if (_PyEval_IsGILEnabled(tstate) && !tstate->_status.holds_gil) {