GH-142513: Reimplement executor management (GH-142931)

* Invalidating an executor does not cause arbitrary code to run
* Executors are only freed at safe points

Author: markshannon

Include/internal/pycore_interp_structs.h

@@ -947,7 +947,6 @@ struct _is {
     struct _PyExecutorObject *executor_deletion_list_head;
     struct _PyExecutorObject *cold_executor;
     struct _PyExecutorObject *cold_dynamic_executor;
-    int executor_deletion_list_remaining_capacity;
     size_t executor_creation_counter;
     _rare_events rare_events;
     PyDict_WatchCallback builtins_dict_watcher;

Include/internal/pycore_optimizer.h

@@ -25,7 +25,6 @@ typedef struct {
     uint8_t opcode;
     uint8_t oparg;
     uint8_t valid;
-    uint8_t linked;
     uint8_t chain_depth;  // Must be big enough for MAX_CHAIN_DEPTH - 1.
     bool warm;
     int32_t index;           // Index of ENTER_EXECUTOR (if code isn't NULL, below).
@@ -55,11 +54,6 @@ typedef struct _PyExecutorObject {
     _PyExitData exits[1];
 } _PyExecutorObject;
 
-/* If pending deletion list gets large enough, then scan,
- * and free any executors that aren't executing
- * i.e. any that aren't a thread's current_executor. */
-#define EXECUTOR_DELETE_LIST_MAX 100
-
 // Export for '_opcode' shared extension (JIT compiler).
 PyAPI_FUNC(_PyExecutorObject*) _Py_GetExecutor(PyCodeObject *code, int offset);
 
@@ -80,7 +74,6 @@ PyAPI_FUNC(void) _Py_Executors_InvalidateCold(PyInterpreterState *interp);
 #else
 #  define _Py_Executors_InvalidateDependency(A, B, C) ((void)0)
 #  define _Py_Executors_InvalidateAll(A, B) ((void)0)
-#  define _Py_Executors_InvalidateCold(A) ((void)0)
 
 #endif
 

Python/ceval_gil.c

@@ -1397,13 +1397,19 @@ _Py_HandlePending(PyThreadState *tstate)
     if ((breaker & _PY_GC_SCHEDULED_BIT) != 0) {
         _Py_unset_eval_breaker_bit(tstate, _PY_GC_SCHEDULED_BIT);
         _Py_RunGC(tstate);
+#ifdef _Py_TIER2
+        _Py_ClearExecutorDeletionList(tstate->interp);
+#endif
     }
 
+#ifdef _Py_TIER2
     if ((breaker & _PY_EVAL_JIT_INVALIDATE_COLD_BIT) != 0) {
         _Py_unset_eval_breaker_bit(tstate, _PY_EVAL_JIT_INVALIDATE_COLD_BIT);
         _Py_Executors_InvalidateCold(tstate->interp);
         tstate->interp->executor_creation_counter = JIT_CLEANUP_THRESHOLD;
+        _Py_ClearExecutorDeletionList(tstate->interp);
     }
+#endif
 
     /* GIL drop request */
     if ((breaker & _PY_GIL_DROP_REQUEST_BIT) != 0) {

Python/optimizer.c

@@ -246,8 +246,6 @@ get_oparg(PyObject *self, PyObject *Py_UNUSED(ignored))
 ///////////////////// Experimental UOp Optimizer /////////////////////
 
 static int executor_clear(PyObject *executor);
-static void unlink_executor(_PyExecutorObject *executor);
-
 
 void
 _PyExecutor_Free(_PyExecutorObject *self)
@@ -258,63 +256,76 @@ _PyExecutor_Free(_PyExecutorObject *self)
     PyObject_GC_Del(self);
 }
 
+static void executor_invalidate(PyObject *op);
+
+static void
+executor_clear_exits(_PyExecutorObject *executor)
+{
+    _PyExecutorObject *cold = _PyExecutor_GetColdExecutor();
+    _PyExecutorObject *cold_dynamic = _PyExecutor_GetColdDynamicExecutor();
+    for (uint32_t i = 0; i < executor->exit_count; i++) {
+        _PyExitData *exit = &executor->exits[i];
+        exit->temperature = initial_unreachable_backoff_counter();
+        _PyExecutorObject *old = executor->exits[i].executor;
+        exit->executor = exit->is_dynamic ? cold_dynamic : cold;
+        Py_DECREF(old);
+    }
+}
+
+
 void
 _Py_ClearExecutorDeletionList(PyInterpreterState *interp)
 {
+    if (interp->executor_deletion_list_head == NULL) {
+        return;
+    }
     _PyRuntimeState *runtime = &_PyRuntime;
     HEAD_LOCK(runtime);
     PyThreadState* ts = PyInterpreterState_ThreadHead(interp);
+    while (ts) {
+        _PyExecutorObject *current = (_PyExecutorObject *)ts->current_executor;
+        Py_XINCREF(current);
+        ts = ts->next;
+    }
     HEAD_UNLOCK(runtime);
+    _PyExecutorObject *keep_list = NULL;
+    do {
+        _PyExecutorObject *exec = interp->executor_deletion_list_head;
+        interp->executor_deletion_list_head = exec->vm_data.links.next;
+        if (Py_REFCNT(exec) == 0) {
+            _PyExecutor_Free(exec);
+        } else {
+            exec->vm_data.links.next = keep_list;
+            keep_list = exec;
+        }
+    } while (interp->executor_deletion_list_head != NULL);
+    interp->executor_deletion_list_head = keep_list;
+    HEAD_LOCK(runtime);
+    ts = PyInterpreterState_ThreadHead(interp);
     while (ts) {
         _PyExecutorObject *current = (_PyExecutorObject *)ts->current_executor;
         if (current != NULL) {
-            /* Anything in this list will be unlinked, so we can reuse the
-             * linked field as a reachability marker. */
-            current->vm_data.linked = 1;
+            _Py_DECREF_NO_DEALLOC((PyObject *)current);
         }
-        HEAD_LOCK(runtime);
-        ts = PyThreadState_Next(ts);
-        HEAD_UNLOCK(runtime);
-    }
-    _PyExecutorObject **prev_to_next_ptr = &interp->executor_deletion_list_head;
-    _PyExecutorObject *exec = *prev_to_next_ptr;
-    while (exec != NULL) {
-        if (exec->vm_data.linked) {
-            // This executor is currently executing
-            exec->vm_data.linked = 0;
-            prev_to_next_ptr = &exec->vm_data.links.next;
-        }
-        else {
-            *prev_to_next_ptr = exec->vm_data.links.next;
-            _PyExecutor_Free(exec);
-        }
-        exec = *prev_to_next_ptr;
+        ts = ts->next;
     }
-    interp->executor_deletion_list_remaining_capacity = EXECUTOR_DELETE_LIST_MAX;
+    HEAD_UNLOCK(runtime);
 }
 
 static void
 add_to_pending_deletion_list(_PyExecutorObject *self)
 {
     PyInterpreterState *interp = PyInterpreterState_Get();
+    self->vm_data.links.previous = NULL;
     self->vm_data.links.next = interp->executor_deletion_list_head;
     interp->executor_deletion_list_head = self;
-    if (interp->executor_deletion_list_remaining_capacity > 0) {
-        interp->executor_deletion_list_remaining_capacity--;
-    }
-    else {
-        _Py_ClearExecutorDeletionList(interp);
-    }
 }
 
 static void
 uop_dealloc(PyObject *op) {
     _PyExecutorObject *self = _PyExecutorObject_CAST(op);
-    _PyObject_GC_UNTRACK(self);
+    executor_invalidate(op);
     assert(self->vm_data.code == NULL);
-    unlink_executor(self);
-    // Once unlinked it becomes impossible to invalidate an executor, so do it here.
-    self->vm_data.valid = 0;
     add_to_pending_deletion_list(self);
 }
 
@@ -1621,19 +1632,14 @@ link_executor(_PyExecutorObject *executor)
         head->vm_data.links.previous = executor;
         interp->executor_list_head = executor;
     }
-    executor->vm_data.linked = true;
     /* executor_list_head must be first in list */
     assert(interp->executor_list_head->vm_data.links.previous == NULL);
 }
 
 static void
 unlink_executor(_PyExecutorObject *executor)
 {
-    if (!executor->vm_data.linked) {
-        return;
-    }
     _PyExecutorLinkListNode *links = &executor->vm_data.links;
-    assert(executor->vm_data.valid);
     _PyExecutorObject *next = links->next;
     _PyExecutorObject *prev = links->previous;
     if (next != NULL) {
@@ -1648,7 +1654,6 @@ unlink_executor(_PyExecutorObject *executor)
         assert(interp->executor_list_head == executor);
         interp->executor_list_head = next;
     }
-    executor->vm_data.linked = false;
 }
 
 /* This must be called by optimizers before using the executor */
@@ -1662,61 +1667,47 @@ _Py_ExecutorInit(_PyExecutorObject *executor, const _PyBloomFilter *dependency_s
     link_executor(executor);
 }
 
-_PyExecutorObject *
-_PyExecutor_GetColdExecutor(void)
+static _PyExecutorObject *
+make_cold_executor(uint16_t opcode)
 {
-    PyInterpreterState *interp = _PyInterpreterState_GET();
-    if (interp->cold_executor != NULL) {
-        return interp->cold_executor;
-    }
     _PyExecutorObject *cold = allocate_executor(0, 1);
     if (cold == NULL) {
         Py_FatalError("Cannot allocate core JIT code");
     }
-    ((_PyUOpInstruction *)cold->trace)->opcode = _COLD_EXIT_r00;
-#ifdef _Py_JIT
-    cold->jit_code = NULL;
-    cold->jit_size = 0;
+    ((_PyUOpInstruction *)cold->trace)->opcode = opcode;
     // This is initialized to true so we can prevent the executor
     // from being immediately detected as cold and invalidated.
     cold->vm_data.warm = true;
+#ifdef _Py_JIT
+    cold->jit_code = NULL;
+    cold->jit_size = 0;
     if (_PyJIT_Compile(cold, cold->trace, 1)) {
         Py_DECREF(cold);
         Py_FatalError("Cannot allocate core JIT code");
     }
 #endif
     _Py_SetImmortal((PyObject *)cold);
-    interp->cold_executor = cold;
     return cold;
 }
 
 _PyExecutorObject *
-_PyExecutor_GetColdDynamicExecutor(void)
+_PyExecutor_GetColdExecutor(void)
 {
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    if (interp->cold_dynamic_executor != NULL) {
-        assert(interp->cold_dynamic_executor->trace[0].opcode == _COLD_DYNAMIC_EXIT_r00);
-        return interp->cold_dynamic_executor;
-    }
-    _PyExecutorObject *cold = allocate_executor(0, 1);
-    if (cold == NULL) {
-        Py_FatalError("Cannot allocate core JIT code");
+    if (interp->cold_executor == NULL) {
+        return interp->cold_executor = make_cold_executor(_COLD_EXIT_r00);;
     }
-    ((_PyUOpInstruction *)cold->trace)->opcode = _COLD_DYNAMIC_EXIT_r00;
-#ifdef _Py_JIT
-    cold->jit_code = NULL;
-    cold->jit_size = 0;
-    // This is initialized to true so we can prevent the executor
-    // from being immediately detected as cold and invalidated.
-    cold->vm_data.warm = true;
-    if (_PyJIT_Compile(cold, cold->trace, 1)) {
-        Py_DECREF(cold);
-        Py_FatalError("Cannot allocate core JIT code");
+    return interp->cold_executor;
+}
+
+_PyExecutorObject *
+_PyExecutor_GetColdDynamicExecutor(void)
+{
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    if (interp->cold_dynamic_executor == NULL) {
+        interp->cold_dynamic_executor = make_cold_executor(_COLD_DYNAMIC_EXIT_r00);
     }
-#endif
-    _Py_SetImmortal((PyObject *)cold);
-    interp->cold_dynamic_executor = cold;
-    return cold;
+    return interp->cold_dynamic_executor;
 }
 
 void
@@ -1755,32 +1746,28 @@ _Py_ExecutorDetach(_PyExecutorObject *executor)
     Py_DECREF(executor);
 }
 
-static int
-executor_clear(PyObject *op)
+/* Executors can be invalidated at any time,
+   even with a stop-the-world lock held.
+   Consequently it must not run arbitrary code,
+   including Py_DECREF with a non-executor. */
+static void
+executor_invalidate(PyObject *op)
 {
     _PyExecutorObject *executor = _PyExecutorObject_CAST(op);
     if (!executor->vm_data.valid) {
-        return 0;
+        return;
     }
-    assert(executor->vm_data.valid == 1);
-    unlink_executor(executor);
     executor->vm_data.valid = 0;
-
-    /* It is possible for an executor to form a reference
-     * cycle with itself, so decref'ing a side exit could
-     * free the executor unless we hold a strong reference to it
-     */
-    _PyExecutorObject *cold = _PyExecutor_GetColdExecutor();
-    Py_INCREF(executor);
-    for (uint32_t i = 0; i < executor->exit_count; i++) {
-        executor->exits[i].temperature = initial_unreachable_backoff_counter();
-        _PyExecutorObject *e = executor->exits[i].executor;
-        executor->exits[i].executor = cold;
-        Py_DECREF(e);
-    }
+    unlink_executor(executor);
+    executor_clear_exits(executor);
     _Py_ExecutorDetach(executor);
-    Py_DECREF(executor);
-    return 0;
+    _PyObject_GC_UNTRACK(op);
+}
+
+static int
+executor_clear(PyObject *op)
+{
+    executor_invalidate(op);
 }
 
 void
@@ -1805,7 +1792,7 @@ _Py_Executors_InvalidateDependency(PyInterpreterState *interp, void *obj, int is
     if (invalidate == NULL) {
         goto error;
     }
-    /* Clearing an executor can deallocate others, so we need to make a list of
+    /* Clearing an executor can clear others, so we need to make a list of
      * executors to invalidate first */
     for (_PyExecutorObject *exec = interp->executor_list_head; exec != NULL;) {
         assert(exec->vm_data.valid);
@@ -1819,7 +1806,7 @@ _Py_Executors_InvalidateDependency(PyInterpreterState *interp, void *obj, int is
     }
     for (Py_ssize_t i = 0; i < PyList_GET_SIZE(invalidate); i++) {
         PyObject *exec = PyList_GET_ITEM(invalidate, i);
-        executor_clear(exec);
+        executor_invalidate(exec);
         if (is_invalidation) {
             OPT_STAT_INC(executors_invalidated);
         }
@@ -1851,13 +1838,13 @@ _Py_Executors_InvalidateAll(PyInterpreterState *interp, int is_invalidation)
 {
     while (interp->executor_list_head) {
         _PyExecutorObject *executor = interp->executor_list_head;
-        assert(executor->vm_data.valid == 1 && executor->vm_data.linked == 1);
+        assert(executor->vm_data.valid);
         if (executor->vm_data.code) {
             // Clear the entire code object so its co_executors array be freed:
             _PyCode_Clear_Executors(executor->vm_data.code);
         }
         else {
-            executor_clear((PyObject *)executor);
+            executor_invalidate((PyObject *)executor);
         }
         if (is_invalidation) {
             OPT_STAT_INC(executors_invalidated);
@@ -1892,7 +1879,7 @@ _Py_Executors_InvalidateCold(PyInterpreterState *interp)
     }
     for (Py_ssize_t i = 0; i < PyList_GET_SIZE(invalidate); i++) {
         PyObject *exec = PyList_GET_ITEM(invalidate, i);
-        executor_clear(exec);
+        executor_invalidate(exec);
     }
     Py_DECREF(invalidate);
     return;

Python/pystate.c

@@ -570,7 +570,6 @@ init_interpreter(PyInterpreterState *interp,
     interp->compiling = false;
     interp->executor_list_head = NULL;
     interp->executor_deletion_list_head = NULL;
-    interp->executor_deletion_list_remaining_capacity = 0;
     interp->executor_creation_counter = JIT_CLEANUP_THRESHOLD;
     if (interp != &runtime->_main_interpreter) {
         /* Fix the self-referential, statically initialized fields. */