gh-112175: Add `eval_breaker` to `PyThreadState` (#115194)

This change adds an `eval_breaker` field to `PyThreadState`. The primary
motivation is for performance in free-threaded builds: with thread-local eval
breakers, we can stop a specific thread (e.g., for an async exception) without
interrupting other threads.

The source of truth for the global instrumentation version is stored in the
`instrumentation_version` field in PyInterpreterState. Threads usually read the
version from their local `eval_breaker`, where it continues to be colocated
with the eval breaker bits.

1 files changed, 7 insertions, 9 deletions

diff --git a/Python/pystate.c b/Python/pystate.c
index 3484bea..4cd975a 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -794,9 +794,7 @@ interpreter_clear(PyInterpreterState *interp, PyThreadState *tstate)
 
     Py_CLEAR(interp->audit_hooks);
 
-    // At this time, all the threads should be cleared so we don't need
-    // atomic operations for eval_breaker
-    interp->ceval.eval_breaker = 0;
+    interp->ceval.instrumentation_version = 0;
 
     for (int i = 0; i < _PY_MONITORING_UNGROUPED_EVENTS; i++) {
         interp->monitors.tools[i] = 0;
@@ -1318,6 +1316,8 @@ init_threadstate(_PyThreadStateImpl *_tstate,
 
     assert(interp != NULL);
     tstate->interp = interp;
+    tstate->eval_breaker =
+        _Py_atomic_load_uintptr_relaxed(&interp->ceval.instrumentation_version);
 
     // next/prev are set in add_threadstate().
     assert(tstate->next == NULL);
@@ -2021,8 +2021,7 @@ park_detached_threads(struct _stoptheworld_state *stw)
             }
         }
         else if (state == _Py_THREAD_ATTACHED && t != stw->requester) {
-            // TODO: set this per-thread, rather than per-interpreter.
-            _Py_set_eval_breaker_bit(t->interp, _PY_EVAL_PLEASE_STOP_BIT, 1);
+            _Py_set_eval_breaker_bit(t, _PY_EVAL_PLEASE_STOP_BIT);
         }
     }
     stw->thread_countdown -= num_parked;
@@ -2186,19 +2185,18 @@ PyThreadState_SetAsyncExc(unsigned long id, PyObject *exc)
          * deadlock, we need to release head_mutex before
          * the decref.
          */
-        PyObject *old_exc = tstate->async_exc;
-        tstate->async_exc = Py_XNewRef(exc);
+        Py_XINCREF(exc);
+        PyObject *old_exc = _Py_atomic_exchange_ptr(&tstate->async_exc, exc);
         HEAD_UNLOCK(runtime);
 
         Py_XDECREF(old_exc);
-        _PyEval_SignalAsyncExc(tstate->interp);
+        _Py_set_eval_breaker_bit(tstate, _PY_ASYNC_EXCEPTION_BIT);
         return 1;
     }
     HEAD_UNLOCK(runtime);
     return 0;
 }
 
-
 //---------------------------------
 // API for the current thread state
 //---------------------------------