pythongh-124878: Fix race conditions during interpreter finalization

The PyThreadState field gains a reference count field to avoid
issues with PyThreadState being a dangling pointer to freed memory.
The refcount starts with a value of two: one reference is owned by the
interpreter's linked list of thread states and one reference is owned by
the OS thread. The reference count is decremented when the thread state
is removed from the interpreter's linked list and before the OS thread
calls `PyThread_hang_thread()`. The thread that decrements it to zero
frees the `PyThreadState` memory.

The `holds_gil` field is moved out of the `_status` bit field, to avoid
a data race where on thread calls `PyThreadState_Clear()`, modifying the
`_status` bit field while the OS thread reads `holds_gil` when
attempting to acquire the GIL.

The `PyThreadState.state` field now has `_Py_THREAD_SHUTTING_DOWN` as a
possible value. This corresponds to the `_PyThreadState_MustExit()`
check. This avoids race conditions in the free threading build when
checking `_PyThreadState_MustExit()`.

Author: colesbury

Include/cpython/pystate.h

@@ -83,16 +83,14 @@ struct _ts {
         unsigned int bound_gilstate:1;
         /* Currently in use (maybe holds the GIL). */
         unsigned int active:1;
-        /* Currently holds the GIL. */
-        unsigned int holds_gil:1;
 
         /* various stages of finalization */
         unsigned int finalizing:1;
         unsigned int cleared:1;
         unsigned int finalized:1;
 
         /* padding to align to 4 bytes */
-        unsigned int :23;
+        unsigned int :24;
     } _status;
 #ifdef Py_BUILD_CORE
 #  define _PyThreadState_WHENCE_NOTSET -1
@@ -103,6 +101,10 @@ struct _ts {
 #  define _PyThreadState_WHENCE_GILSTATE 4
 #  define _PyThreadState_WHENCE_EXEC 5
 #endif
+
+    /* Currently holds the GIL. Must be its own field to avoid data races */
+    int holds_gil;
+
     int _whence;
 
     /* Thread state (_Py_THREAD_ATTACHED, _Py_THREAD_DETACHED, _Py_THREAD_SUSPENDED).

Include/internal/pycore_pystate.h

@@ -27,6 +27,10 @@ extern "C" {
 // "suspended" state. Only the thread performing a stop-the-world pause may
 // transition a thread from the "suspended" state back to the "detached" state.
 //
+// The "shutting down" state is used when the interpreter is being finalized.
+// Threads in this state can't do anything other than block the OS thread.
+// (See _PyThreadState_HangThread).
+//
 // State transition diagram:
 //
 //            (bound thread)        (stop-the-world thread)
@@ -37,9 +41,10 @@ extern "C" {
 //
 // The (bound thread) and (stop-the-world thread) labels indicate which thread
 // is allowed to perform the transition.
-#define _Py_THREAD_DETACHED     0
-#define _Py_THREAD_ATTACHED     1
-#define _Py_THREAD_SUSPENDED    2
+#define _Py_THREAD_DETACHED         0
+#define _Py_THREAD_ATTACHED         1
+#define _Py_THREAD_SUSPENDED        2
+#define _Py_THREAD_SHUTTING_DOWN    3
 
 
 /* Check if the current thread is the main thread.
@@ -118,7 +123,8 @@ extern _Py_thread_local PyThreadState *_Py_tss_tstate;
 extern int _PyThreadState_CheckConsistency(PyThreadState *tstate);
 #endif
 
-int _PyThreadState_MustExit(PyThreadState *tstate);
+extern int _PyThreadState_MustExit(PyThreadState *tstate);
+extern void _PyThreadState_HangThread(PyThreadState *tstate);
 
 // Export for most shared extensions, used via _PyThreadState_GET() static
 // inline function.
@@ -169,6 +175,11 @@ extern void _PyThreadState_Detach(PyThreadState *tstate);
 // to the "detached" state.
 extern void _PyThreadState_Suspend(PyThreadState *tstate);
 
+// Mark the thread state as "shutting down". This is used during interpreter
+// and runtime finalization. The thread may no longer attach to the
+// interpreter and will instead block via _PyThreadState_HangThread().
+extern void _PyThreadState_SetShuttingDown(PyThreadState *tstate);
+
 // Perform a stop-the-world pause for all threads in the all interpreters.
 //
 // Threads in the "attached" state are paused and transitioned to the "GC"
@@ -238,7 +249,7 @@ PyAPI_FUNC(PyThreadState *) _PyThreadState_NewBound(
     PyInterpreterState *interp,
     int whence);
 extern PyThreadState * _PyThreadState_RemoveExcept(PyThreadState *tstate);
-extern void _PyThreadState_DeleteList(PyThreadState *list);
+extern void _PyThreadState_DeleteList(PyThreadState *list, int is_after_fork);
 extern void _PyThreadState_ClearMimallocHeaps(PyThreadState *tstate);
 
 // Export for '_testinternalcapi' shared extension

Include/internal/pycore_runtime_init.h

@@ -171,6 +171,8 @@ extern PyTypeObject _PyExc_MemoryError;
 #define _PyThreadStateImpl_INIT \
     { \
         .base = _PyThreadState_INIT, \
+        /* The thread and the interpreter's linked list hold a reference */ \
+        .refcount = 2, \
     }
 
 #define _PyThreadState_INIT \

Include/internal/pycore_tstate.h

@@ -21,6 +21,10 @@ typedef struct _PyThreadStateImpl {
     // semi-public fields are in PyThreadState.
     PyThreadState base;
 
+    // The reference count field is used to synchronize deallocation of the
+    // thread state during runtime finalization.
+    Py_ssize_t refcount;
+
     // These are addresses, but we need to convert to ints to avoid UB.
     uintptr_t c_stack_top;
     uintptr_t c_stack_soft_limit;

Misc/NEWS.d/next/Core_and_Builtins/2025-02-27-18-48-42.gh-issue-124878.DS0MIL.rst

@@ -0,0 +1,2 @@
+Fix race conditions during runtime finalization that could lead to accessing
+freed memory.

Modules/_threadmodule.c

@@ -333,9 +333,6 @@ thread_run(void *boot_raw)
     // _PyRuntimeState_SetFinalizing() is called. At this point, all Python
     // threads must exit, except of the thread calling Py_Finalize() which
     // holds the GIL and must not exit.
-    //
-    // At this stage, tstate can be a dangling pointer (point to freed memory),
-    // it's ok to call _PyThreadState_MustExit() with a dangling pointer.
     if (_PyThreadState_MustExit(tstate)) {
         // Don't call PyThreadState_Clear() nor _PyThreadState_DeleteCurrent().
         // These functions are called on tstate indirectly by Py_Finalize()

Modules/posixmodule.c

@@ -715,7 +715,7 @@ PyOS_AfterFork_Child(void)
     // may call destructors.
     PyThreadState *list = _PyThreadState_RemoveExcept(tstate);
     _PyEval_StartTheWorldAll(&_PyRuntime);
-    _PyThreadState_DeleteList(list);
+    _PyThreadState_DeleteList(list, /*is_after_fork=*/1);
 
     _PyImport_ReInitLock(tstate->interp);
     _PyImport_ReleaseLock(tstate->interp);

Python/ceval_gil.c

@@ -6,8 +6,8 @@
 #include "pycore_pyerrors.h"      // _PyErr_GetRaisedException()
 #include "pycore_pylifecycle.h"   // _PyErr_Print()
 #include "pycore_pymem.h"         // _PyMem_IsPtrFreed()
+#include "pycore_pystate.h"       // PyThread_hang_thread()
 #include "pycore_pystats.h"       // _Py_PrintSpecializationStats()
-#include "pycore_pythread.h"      // PyThread_hang_thread()
 
 /*
    Notes about the implementation:
@@ -206,7 +206,7 @@ drop_gil_impl(PyThreadState *tstate, struct _gil_runtime_state *gil)
     _Py_ANNOTATE_RWLOCK_RELEASED(&gil->locked, /*is_write=*/1);
     _Py_atomic_store_int_relaxed(&gil->locked, 0);
     if (tstate != NULL) {
-        tstate->_status.holds_gil = 0;
+        tstate->holds_gil = 0;
     }
     COND_SIGNAL(gil->cond);
     MUTEX_UNLOCK(gil->mutex);
@@ -231,7 +231,7 @@ drop_gil(PyInterpreterState *interp, PyThreadState *tstate, int final_release)
     // Check if we have the GIL before dropping it. tstate will be NULL if
     // take_gil() detected that this thread has been destroyed, in which case
     // we know we have the GIL.
-    if (tstate != NULL && !tstate->_status.holds_gil) {
+    if (tstate != NULL && !tstate->holds_gil) {
         return;
     }
 #endif
@@ -296,15 +296,14 @@ take_gil(PyThreadState *tstate)
            thread which called Py_Finalize(), this thread cannot continue.
 
            This code path can be reached by a daemon thread after Py_Finalize()
-           completes. In this case, tstate is a dangling pointer: points to
-           PyThreadState freed memory.
+           completes.
 
            This used to call a *thread_exit API, but that was not safe as it
            lacks stack unwinding and local variable destruction important to
            C++. gh-87135: The best that can be done is to hang the thread as
            the public APIs calling this have no error reporting mechanism (!).
          */
-        PyThread_hang_thread();
+        _PyThreadState_HangThread(tstate);
     }
 
     assert(_PyThreadState_CheckConsistency(tstate));
@@ -350,7 +349,7 @@ take_gil(PyThreadState *tstate)
                 }
                 // gh-87135: hang the thread as *thread_exit() is not a safe
                 // API. It lacks stack unwind and local variable destruction.
-                PyThread_hang_thread();
+                _PyThreadState_HangThread(tstate);
             }
             assert(_PyThreadState_CheckConsistency(tstate));
 
@@ -401,11 +400,11 @@ take_gil(PyThreadState *tstate)
         /* tstate could be a dangling pointer, so don't pass it to
            drop_gil(). */
         drop_gil(interp, NULL, 1);
-        PyThread_hang_thread();
+        _PyThreadState_HangThread(tstate);
     }
     assert(_PyThreadState_CheckConsistency(tstate));
 
-    tstate->_status.holds_gil = 1;
+    tstate->holds_gil = 1;
     _Py_unset_eval_breaker_bit(tstate, _PY_GIL_DROP_REQUEST_BIT);
     update_eval_breaker_for_thread(interp, tstate);
 
@@ -457,7 +456,7 @@ PyEval_ThreadsInitialized(void)
 static inline int
 current_thread_holds_gil(struct _gil_runtime_state *gil, PyThreadState *tstate)
 {
-    int holds_gil = tstate->_status.holds_gil;
+    int holds_gil = tstate->holds_gil;
 
     // holds_gil is the source of truth; check that last_holder and gil->locked
     // are consistent with it.

Python/pylifecycle.c

@@ -2036,18 +2036,23 @@ _Py_Finalize(_PyRuntimeState *runtime)
 
     // XXX Call something like _PyImport_Disable() here?
 
-    /* Destroy the state of all threads of the interpreter, except of the
+    /* Remove the state of all threads of the interpreter, except for the
        current thread. In practice, only daemon threads should still be alive,
        except if wait_for_thread_shutdown() has been cancelled by CTRL+C.
-       Clear frames of other threads to call objects destructors. Destructors
-       will be called in the current Python thread. Since
-       _PyRuntimeState_SetFinalizing() has been called, no other Python thread
-       can take the GIL at this point: if they try, they will exit
-       immediately. We start the world once we are the only thread state left,
+       We start the world once we are the only thread state left,
        before we call destructors. */
     PyThreadState *list = _PyThreadState_RemoveExcept(tstate);
+    for (PyThreadState *p = list; p != NULL; p = p->next) {
+        _PyThreadState_SetShuttingDown(p);
+    }
     _PyEval_StartTheWorldAll(runtime);
-    _PyThreadState_DeleteList(list);
+
+    /* Clear frames of other threads to call objects destructors. Destructors
+       will be called in the current Python thread. Since
+       _PyRuntimeState_SetFinalizing() has been called, no other Python thread
+       can take the GIL at this point: if they try, they will hang in
+       _PyThreadState_HangThread. */
+    _PyThreadState_DeleteList(list, /*is_after_fork=*/0);
 
     /* At this point no Python code should be running at all.
        The only thread state left should be the main thread of the main

Python/pystate.c

@@ -1455,6 +1455,15 @@ free_threadstate(_PyThreadStateImpl *tstate)
     }
 }
 
+static void
+decref_threadstate(_PyThreadStateImpl *tstate)
+{
+    if (_Py_atomic_add_ssize(&tstate->refcount, -1) == 1) {
+        // The last reference to the thread state is gone.
+        free_threadstate(tstate);
+    }
+}
+
 /* Get the thread state to a minimal consistent state.
    Further init happens in pylifecycle.c before it can be used.
    All fields not initialized here are expected to be zeroed out,
@@ -1919,8 +1928,12 @@ _PyThreadState_RemoveExcept(PyThreadState *tstate)
 // Deletes the thread states in the linked list `list`.
 //
 // This is intended to be used in conjunction with _PyThreadState_RemoveExcept.
+//
+// If `is_after_fork` is true, the thread states are immediately freed.
+// Otherwise, they are decref'd because they may still be referenced by an
+// OS thread.
 void
-_PyThreadState_DeleteList(PyThreadState *list)
+_PyThreadState_DeleteList(PyThreadState *list, int is_after_fork)
 {
     // The world can't be stopped because we PyThreadState_Clear() can
     // call destructors.
@@ -1930,7 +1943,12 @@ _PyThreadState_DeleteList(PyThreadState *list)
     for (p = list; p; p = next) {
         next = p->next;
         PyThreadState_Clear(p);
-        free_threadstate((_PyThreadStateImpl *)p);
+        if (is_after_fork) {
+            free_threadstate((_PyThreadStateImpl *)p);
+        }
+        else {
+            decref_threadstate((_PyThreadStateImpl *)p);
+        }
     }
 }
 
@@ -2063,12 +2081,19 @@ static void
 tstate_wait_attach(PyThreadState *tstate)
 {
     do {
-        int expected = _Py_THREAD_SUSPENDED;
-
-        // Wait until we're switched out of SUSPENDED to DETACHED.
-        _PyParkingLot_Park(&tstate->state, &expected, sizeof(tstate->state),
-                           /*timeout=*/-1, NULL, /*detach=*/0);
-
+        int state = _Py_atomic_load_int_relaxed(&tstate->state);
+        if (state == _Py_THREAD_SUSPENDED) {
+            // Wait until we're switched out of SUSPENDED to DETACHED.
+            _PyParkingLot_Park(&tstate->state, &state, sizeof(tstate->state),
+                               /*timeout=*/-1, NULL, /*detach=*/0);
+        }
+        else if (state == _Py_THREAD_SHUTTING_DOWN) {
+            // We're shutting down, so we can't attach.
+            _PyThreadState_HangThread(tstate);
+        }
+        else {
+            assert(state == _Py_THREAD_DETACHED);
+        }
         // Once we're back in DETACHED we can re-attach
     } while (!tstate_try_attach(tstate));
 }
@@ -2100,7 +2125,7 @@ _PyThreadState_Attach(PyThreadState *tstate)
         }
 
 #ifdef Py_GIL_DISABLED
-        if (_PyEval_IsGILEnabled(tstate) && !tstate->_status.holds_gil) {
+        if (_PyEval_IsGILEnabled(tstate) && !tstate->holds_gil) {
             // The GIL was enabled between our call to _PyEval_AcquireLock()
             // and when we attached (the GIL can't go from enabled to disabled
             // here because only a thread holding the GIL can disable
@@ -2183,6 +2208,15 @@ _PyThreadState_Suspend(PyThreadState *tstate)
     HEAD_UNLOCK(runtime);
 }
 
+void
+_PyThreadState_SetShuttingDown(PyThreadState *tstate)
+{
+    _Py_atomic_store_int(&tstate->state, _Py_THREAD_SHUTTING_DOWN);
+#ifdef Py_GIL_DISABLED
+    _PyParkingLot_UnparkAll(&tstate->state);
+#endif
+}
+
 // Decrease stop-the-world counter of remaining number of threads that need to
 // pause. If we are the final thread to pause, notify the requesting thread.
 static void
@@ -2983,43 +3017,27 @@ _PyThreadState_CheckConsistency(PyThreadState *tstate)
 #endif
 
 
-// Check if a Python thread must exit immediately, rather than taking the GIL
-// if Py_Finalize() has been called.
+// Check if a Python thread must call _PyThreadState_HangThread(), rather than
+// taking the GIL or attaching to the interpreter if Py_Finalize() has been
+// called.
 //
 // When this function is called by a daemon thread after Py_Finalize() has been
-// called, the GIL does no longer exist.
-//
-// tstate can be a dangling pointer (point to freed memory): only tstate value
-// is used, the pointer is not deferenced.
+// called, the GIL may no longer exist.
 //
 // tstate must be non-NULL.
 int
 _PyThreadState_MustExit(PyThreadState *tstate)
 {
-    /* bpo-39877: Access _PyRuntime directly rather than using
-       tstate->interp->runtime to support calls from Python daemon threads.
-       After Py_Finalize() has been called, tstate can be a dangling pointer:
-       point to PyThreadState freed memory. */
-    unsigned long finalizing_id = _PyRuntimeState_GetFinalizingID(&_PyRuntime);
-    PyThreadState *finalizing = _PyRuntimeState_GetFinalizing(&_PyRuntime);
-    if (finalizing == NULL) {
-        // XXX This isn't completely safe from daemon thraeds,
-        // since tstate might be a dangling pointer.
-        finalizing = _PyInterpreterState_GetFinalizing(tstate->interp);
-        finalizing_id = _PyInterpreterState_GetFinalizingID(tstate->interp);
-    }
-    // XXX else check &_PyRuntime._main_interpreter._initial_thread
-    if (finalizing == NULL) {
-        return 0;
-    }
-    else if (finalizing == tstate) {
-        return 0;
-    }
-    else if (finalizing_id == PyThread_get_thread_ident()) {
-        /* gh-109793: we must have switched interpreters. */
-        return 0;
-    }
-    return 1;
+    int state = _Py_atomic_load_int_relaxed(&tstate->state);
+    return state == _Py_THREAD_SHUTTING_DOWN;
+}
+
+void
+_PyThreadState_HangThread(PyThreadState *tstate)
+{
+    _PyThreadStateImpl *tstate_impl = (_PyThreadStateImpl *)tstate;
+    decref_threadstate(tstate_impl);
+    PyThread_hang_thread();
 }
 
 /********************/

Python/qsbr.c

@@ -240,7 +240,7 @@ _Py_qsbr_unregister(PyThreadState *tstate)
     // gh-119369: GIL must be released (if held) to prevent deadlocks, because
     // we might not have an active tstate, which means that blocking on PyMutex
     // locks will not implicitly release the GIL.
-    assert(!tstate->_status.holds_gil);
+    assert(!tstate->holds_gil);
 
     PyMutex_Lock(&shared->mutex);
     // NOTE: we must load (or reload) the thread state's qbsr inside the mutex