[Python-checkins] bpo-39877: Fix PyEval_RestoreThread() for daemon threads (GH-18811)

Sun Mar 8 06:57:50 EDT 2020

https://github.com/python/cpython/commit/eb4e2ae2b8486e8ee4249218b95d94a9f0cc513e
commit: eb4e2ae2b8486e8ee4249218b95d94a9f0cc513e
branch: master
author: Victor Stinner <vstinner at python.org>
committer: GitHub <noreply at github.com>
date: 2020年03月08日T11:57:45+01:00
summary:
bpo-39877: Fix PyEval_RestoreThread() for daemon threads (GH-18811)
* exit_thread_if_finalizing() does now access directly _PyRuntime
 variable, rather than using tstate->interp->runtime since tstate
 can be a dangling pointer after Py_Finalize() has been called.
* exit_thread_if_finalizing() is now called *before* calling
 take_gil(). _PyRuntime.finalizing is an atomic variable,
 we don't need to hold the GIL to access it.
* Add ensure_tstate_not_null() function to check that tstate is not
 NULL at runtime. Check tstate earlier. take_gil() does not longer
 check if tstate is NULL.
Cleanup:
* PyEval_RestoreThread() no longer saves/restores errno: it's already
 done inside take_gil().
* PyEval_AcquireLock(), PyEval_AcquireThread(),
 PyEval_RestoreThread() and _PyEval_EvalFrameDefault() now check if
 tstate is valid with the new is_tstate_valid() function which uses
 _PyMem_IsPtrFreed().
files:
A Misc/NEWS.d/next/Core and Builtins/2020-03-06-18-30-00.bpo-39877.bzd1y0.rst
M Python/ceval.c
M Python/ceval_gil.h
M Python/pylifecycle.c

diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-03-06-18-30-00.bpo-39877.bzd1y0.rst b/Misc/NEWS.d/next/Core and Builtins/2020-03-06-18-30-00.bpo-39877.bzd1y0.rst
new file mode 100644
index 0000000000000..d545813c1075d
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2020-03-06-18-30-00.bpo-39877.bzd1y0.rst	
@@ -0,0 +1,5 @@
+Fix :c:func:`PyEval_RestoreThread` random crash at exit with daemon threads.
+It now accesses the ``_PyRuntime`` variable directly instead of using
+``tstate->interp->runtime``, since ``tstate`` can be a dangling pointer after
+:c:func:`Py_Finalize` has been called. Moreover, the daemon thread now exits
+before trying to take the GIL.
diff --git a/Python/ceval.c b/Python/ceval.c
index 04e0824727e6e..42f08c4534c64 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -188,6 +188,25 @@ static size_t opcache_global_misses = 0;
 #include "pythread.h"
 #include "ceval_gil.h"
 
+static void
+ensure_tstate_not_null(const char *func, PyThreadState *tstate)
+{
+ if (tstate == NULL) {
+ _Py_FatalErrorFunc(func, "current thread state is NULL");
+ }
+}
+
+
+#ifndef NDEBUG
+static int is_tstate_valid(PyThreadState *tstate)
+{
+ assert(!_PyMem_IsPtrFreed(tstate));
+ assert(!_PyMem_IsPtrFreed(tstate->interp));
+ return 1;
+}
+#endif
+
+
 int
 PyEval_ThreadsInitialized(void)
 {
@@ -208,6 +227,7 @@ PyEval_InitThreads(void)
 PyThread_init_thread();
 create_gil(gil);
 PyThreadState *tstate = _PyRuntimeState_GetThreadState(runtime);
+ ensure_tstate_not_null(__func__, tstate);
 take_gil(ceval, tstate);
 
 struct _pending_calls *pending = &ceval->pending;
@@ -235,14 +255,26 @@ _PyEval_FiniThreads(struct _ceval_runtime_state *ceval)
 }
 }
 
+/* This function is designed to exit daemon threads immediately rather than
+ taking the GIL if Py_Finalize() has been called.
+
+ The caller must *not* hold the GIL, since this function does not release
+ the GIL before exiting the thread.
+
+ When this function is called by a daemon thread after Py_Finalize() has been
+ called, the GIL does no longer exist.
+
+ tstate must be non-NULL. */
 static inline void
 exit_thread_if_finalizing(PyThreadState *tstate)
 {
- _PyRuntimeState *runtime = tstate->interp->runtime;
- /* _Py_Finalizing is protected by the GIL */
+ /* 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. */
+ _PyRuntimeState *runtime = &_PyRuntime;
 PyThreadState *finalizing = _PyRuntimeState_GetFinalizing(runtime);
 if (finalizing != NULL && finalizing != tstate) {
- drop_gil(&runtime->ceval, tstate);
 PyThread_exit_thread();
 }
 }
@@ -280,13 +312,14 @@ void
 PyEval_AcquireLock(void)
 {
 _PyRuntimeState *runtime = &_PyRuntime;
- struct _ceval_runtime_state *ceval = &runtime->ceval;
 PyThreadState *tstate = _PyRuntimeState_GetThreadState(runtime);
- if (tstate == NULL) {
- Py_FatalError("current thread state is NULL");
- }
- take_gil(ceval, tstate);
+ ensure_tstate_not_null(__func__, tstate);
+
 exit_thread_if_finalizing(tstate);
+ assert(is_tstate_valid(tstate));
+
+ struct _ceval_runtime_state *ceval = &runtime->ceval;
+ take_gil(ceval, tstate);
 }
 
 void
@@ -304,15 +337,18 @@ PyEval_ReleaseLock(void)
 void
 PyEval_AcquireThread(PyThreadState *tstate)
 {
- assert(tstate != NULL);
+ ensure_tstate_not_null(__func__, tstate);
+
+ exit_thread_if_finalizing(tstate);
+ assert(is_tstate_valid(tstate));
 
 _PyRuntimeState *runtime = tstate->interp->runtime;
 struct _ceval_runtime_state *ceval = &runtime->ceval;
 
 /* Check someone has called PyEval_InitThreads() to create the lock */
 assert(gil_created(&ceval->gil));
+
 take_gil(ceval, tstate);
- exit_thread_if_finalizing(tstate);
 if (_PyThreadState_Swap(&runtime->gilstate, tstate) != NULL) {
 Py_FatalError("non-NULL old thread state");
 }
@@ -344,8 +380,9 @@ _PyEval_ReInitThreads(_PyRuntimeState *runtime)
 return;
 }
 recreate_gil(&ceval->gil);
- PyThreadState *current_tstate = _PyRuntimeState_GetThreadState(runtime);
- take_gil(ceval, current_tstate);
+ PyThreadState *tstate = _PyRuntimeState_GetThreadState(runtime);
+ ensure_tstate_not_null(__func__, tstate);
+ take_gil(ceval, tstate);
 
 struct _pending_calls *pending = &ceval->pending;
 pending->lock = PyThread_allocate_lock();
@@ -354,7 +391,7 @@ _PyEval_ReInitThreads(_PyRuntimeState *runtime)
 }
 
 /* Destroy all threads except the current one */
- _PyThreadState_DeleteExcept(runtime, current_tstate);
+ _PyThreadState_DeleteExcept(runtime, tstate);
 }
 
 /* This function is used to signal that async exceptions are waiting to be
@@ -383,16 +420,16 @@ PyEval_SaveThread(void)
 void
 PyEval_RestoreThread(PyThreadState *tstate)
 {
- assert(tstate != NULL);
+ ensure_tstate_not_null(__func__, tstate);
+
+ exit_thread_if_finalizing(tstate);
+ assert(is_tstate_valid(tstate));
 
 _PyRuntimeState *runtime = tstate->interp->runtime;
 struct _ceval_runtime_state *ceval = &runtime->ceval;
 assert(gil_created(&ceval->gil));
 
- int err = errno;
 take_gil(ceval, tstate);
- exit_thread_if_finalizing(tstate);
- errno = err;
 
 _PyThreadState_Swap(&runtime->gilstate, tstate);
 }
@@ -750,11 +787,14 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
 PyObject **fastlocals, **freevars;
 PyObject *retval = NULL; /* Return value */
 _PyRuntimeState * const runtime = &_PyRuntime;
- PyThreadState * const tstate = _PyRuntimeState_GetThreadState(runtime);
 struct _ceval_runtime_state * const ceval = &runtime->ceval;
 _Py_atomic_int * const eval_breaker = &ceval->eval_breaker;
 PyCodeObject *co;
 
+ PyThreadState * const tstate = _PyRuntimeState_GetThreadState(runtime);
+ ensure_tstate_not_null(__func__, tstate);
+ assert(is_tstate_valid(tstate));
+
 /* when tracing we set things up so that
 
 not (instr_lb <= current_bytecode_offset < instr_ub)
@@ -1242,11 +1282,11 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
 
 /* Other threads may run now */
 
- take_gil(ceval, tstate);
-
 /* Check if we should make a quick exit. */
 exit_thread_if_finalizing(tstate);
 
+ take_gil(ceval, tstate);
+
 if (_PyThreadState_Swap(&runtime->gilstate, tstate) != NULL) {
 Py_FatalError("orphan tstate");
 }
diff --git a/Python/ceval_gil.h b/Python/ceval_gil.h
index 34d48c990c447..99d576d561517 100644
--- a/Python/ceval_gil.h
+++ b/Python/ceval_gil.h
@@ -180,15 +180,17 @@ drop_gil(struct _ceval_runtime_state *ceval, PyThreadState *tstate)
 #endif
 }
 
+/* Take the GIL.
+
+ The function saves errno at entry and restores its value at exit.
+
+ tstate must be non-NULL. */
 static void
 take_gil(struct _ceval_runtime_state *ceval, PyThreadState *tstate)
 {
- if (tstate == NULL) {
- Py_FatalError("take_gil: NULL tstate");
- }
+ int err = errno;
 
 struct _gil_runtime_state *gil = &ceval->gil;
- int err = errno;
 MUTEX_LOCK(gil->mutex);
 
 if (!_Py_atomic_load_relaxed(&gil->locked)) {
@@ -240,6 +242,7 @@ take_gil(struct _ceval_runtime_state *ceval, PyThreadState *tstate)
 }
 
 MUTEX_UNLOCK(gil->mutex);
+
 errno = err;
 }
 
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index 9e3b25727945a..eaae5fdbb6692 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -1364,8 +1364,8 @@ Py_FinalizeEx(void)
 int malloc_stats = interp->config.malloc_stats;
 #endif
 
- /* Remaining threads (e.g. daemon threads) will automatically exit
- after taking the GIL (in PyEval_RestoreThread()). */
+ /* Remaining daemon threads will automatically exit
+ when they attempt to take the GIL (ex: PyEval_RestoreThread()). */
 _PyRuntimeState_SetFinalizing(runtime, tstate);
 runtime->initialized = 0;
 runtime->core_initialized = 0;
