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python - 在 CPython C 扩展类型上的属性读/写之前获取锁

问题描述

我正在为 CPython 编写一个多线程 C 扩展。我有 POD，它使用简单的libuv rwlock使线程安全。我想包装它，以便可以分配它并通过简单PyMemberDef的从 Python 访问数据。我的问题是，仅仅获取/释放锁定就足够了getattro，setattro还是我错过了什么？我是 CPython API 的新手，所以请随意推荐一种完全不同的方法。

我正在尝试做的简化示例：

#include <Python.h>
#include <uv.h>

typedef struct Pod_s {
  uv_rwlock_t lock;
  int number;
} Pod;

typedef struct PyPod_s {
    PyObject_HEAD
    Pod pod;
} PyPod;

static PyMemberDef[] PyPod_members = {
  {"number", T_INT, offsetof(PyPod, pod) + offsetof(Pod, number)},
  {0},
};

static PyObject *PyPod_new(PyTypeObject *type, PyObject *args, PyObject *kwds) {
  PyPod *self;
  self = (PyPod *) type->tp_alloc(type, 0);
  if(self != NULL)
    uv_rwlock_init(&self->pod.lock);  // In real life this would be error-checked
  return (PyObject *) self;
}

// Is this getattro/setattro safe?
static PyObject *PyPod_getattro(PyObject *self, PyObject *attr) {
  PyPod *pypod = (PyPod *) self;
  uv_rwlock_rdlock(&pypod->pod.lock);
  PyObject *ret = PyObject_GenericGetAttr(self, attr);
  uv_rwlock_rdunlock(&pypod->pod.lock);
  return ret;
}

static int PyPod_setattro(PyObject *self, PyObject *attr, PyObject *value) {
  PyPod *pypod = (PyPod *) self;
  uv_rwlock_wrlock(&pypod->pod.lock);
  int ret = PyObject_GenericSetAttr(self, attr, value);
  uv_rwlock_wrunlock(&pypod->pod.lock);
  return ret;
}

static PyTypeObject PyPodType = {
  PyObject_HEAD_INIT(NULL)
  .tp_name = "PodModule.Pod",
  .tp_basicsize = sizeof(PyPod),
  .tp_flags = Py_TPFLAGS_DEFAULT,
  .tp_new = PyPod_new,
  .tp_members = PyPod_members,
  .tp_getattro = PyPod_getattro,
  .tp_setattro = PyPod_setattro,
};

标签： pythoncmultithreadingcpython

解决方案

万一其他人尝试这样做：是的，这对您的 C 代码来说是线程安全的。

The un-asked question here is what about the Python interpreter? If only the main thread (the interpreter that called into your extension) ever calls back into/returns to the interpreter, then you don't have anything to worry about.

If however threads spawned in your C extension have the ability to call into the interpreter, you need to consider the GIL. First release the GIL and acquire the main thread's state with PyEval_SaveThread(). Then use the interp * stored in the thread state to spin up new thread states with PyThreadState_New(interp), one for each thread you intend to create in C. Finally, before calling back into the Python interpreter, acquire the GIL with PyEval_RestoreThread(tstate), call the Python, and release it with PyEval_SaveThread() when you're done.

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