Killing a Stackless bug

What follows is an account of how I found and fixed an insidious bug in Stackless Python which has been there for years.  It’s one of those war stories.  Perhaps a bit long winded and technical and full of exaggerations as such stories tend to be.

Background

Some weeks ago, because of a problem in the client library we are using, I had to switch the http library we are using on the PS3 from using non-blocking IO to blocking. Previously, we were were issuing all the non-blocking calls, the “select” and the tasklet blocking / scheduling on the main thread. This is similar to how gevent and other such libraries do things. Switching to blocking calls, however, meant doing things on worker threads.

The approach we took was to implement a small pool of pyton workers which could execute arbitrary jobs. A new utility function, stacklesslib.util.call_async() then performed the asynchronous call by dispatching it to a worker thread. The idea of an call_async() is to have a different tasklet execute the callable while the caller blocks on a channel. The return value, or error, is then propagated to the originating tasklet using that channel. Stackless channels can be used to communicate between threads too. And synchronizing threads in stackless is even more conveninent than regular Python because there is stackless.atomic, which not only prevents involuntary scheduling of tasklets, it also prevents automatic yielding of the GIL (cPython folks, take note!)

This worked well, and has been running for some time. The drawback to this approach, of course, is that we now need to keep python threads around, consuming stack space. And Python needs a lot of stack.

The problem

The only problem was, that there appeared to be a bug present. One of our developers complained that sometimes, during long downloads, the http download function would return None, rather than the expected string chunk.

Now, this problem was hard to reproduce. It required a specific setup and geolocation was also an issue. This developer is in California, using servers in London. Hence, there ensued a somewhat prolonged interaction (hindered by badly overlapping time-zones) where I would provide him with modified .py files with instrumentation, and he would provide me with logs. We quickly determined, to my dismay, that apparently, sometimes a string was turning into None, while in transit trough a channel.send() to a channel.receive(). This was most distressing. Particularly because the channel in question was transporting data between threads and this particular functionality of stackless has not been as heavily used as the rest.

Tracking it down

So, I suspected a race condition of some sorts. But a careful review of the channel code and the scheduling code presented no obvious candidates. Also, the somehwat unpopular GIL was being used throughout, which if done correctly ensures that things work as expected.

To cut a long story short, by a lucky coincidence I managed to reproduce a different manifestation of the problem. In some cases, a simple interaction with a local HTTP server would cause this to happen.

When a channel sends data between tasklets, it is temporarily stored on the target tasklet’s “tempval” attribute. When the target wakes up, this is then taken and returned as the result from the “receive()” call. I was able to establish that after sending the data, the target tasklet did indeed hold the correct string value in its “tempval” attribute. I then needed to find out where and why it was disappearing from that place.

By adding instrumentation code to the stackless core, I established that this was happening in the last line of the following snippet:

PyObject *
slp_run_tasklet(void)
{
    PyThreadState *ts = PyThreadState_GET();
    PyObject *retval;

    if ( (ts->st.main == NULL) && initialize_main_and_current()) {
        ts->frame = NULL;
        return NULL;
    }

    TASKLET_CLAIMVAL(ts->st.current, &retval);

By setting a breakpoint, I was able to see that I was in the top level part of the “continue” bit of the “stack spilling” code

Stack spilling is a feature of stackless where the stack slicing mechanism is used to recycle a deep callstack. When it detects that the stack has grown beyond a certain limit, it is stored away, and a hard switch is done to the top again, where it continues its downwards crawl. This can help conserve stack address space, particularly on threads where the stack cannot grow dynamically.

So, something wrong with stack spilling, then.  But even so, this was unexpected. Why was stack spilling happening when data was being transmitted across a channel? Stack spilling normally occurs only when nesting regular .py code and other such things.

By setting a breakpoint at the right place, where the stack spilling code was being invoked, I finally arrived at this callstack:

Type Function
PyObject* slp_eval_frame_newstack(PyFrameObject* f, int exc, PyObject* retval)
PyObject* PyEval_EvalFrameEx_slp(PyFrameObject* f, int throwflag, PyObject* retval)
PyObject* slp_frame_dispatch(PyFrameObject* f, PyFrameObject* stopframe, int exc, PyObject* retval)
PyObject* PyEval_EvalCodeEx(PyCodeObject* co, PyObject* globals, PyObject* locals, PyObject** args, int argcount, PyObject** kws, int kwcount, PyObject** defs, int defcount, PyObject* closure)
PyObject* function_call(PyObject* func, PyObject* arg, PyObject* kw)
PyObject* PyObject_Call(PyObject* func, PyObject* arg, PyObject* kw)
PyObject* PyObject_CallFunctionObjArgs(PyObject* callable)
void PyObject_ClearWeakRefs(PyObject* object)
void tasklet_dealloc(PyTaskletObject* t)
void subtype_dealloc(PyObject* self)
int slp_transfer(PyCStackObject** cstprev, PyCStackObject* cst, PyTaskletObject* prev)
PyObject* slp_schedule_task(PyTaskletObject* prev, PyTaskletObject* next, int stackless, int* did_switch)
PyObject* generic_channel_action(PyChannelObject* self, PyObject* arg, int dir, int stackless)
PyObject* impl_channel_receive(PyChannelObject* self)
PyObject* call_function(PyObject*** pp_stack, int oparg)

Notice the “subtype_dealloc”. This callstack indicates that in the channel receive code, after the hard switch back to the target tasklet, a Py_DECREF was causing side effects, which again caused stack spilling to occur. The place was this, in slp_transfer():

/* release any objects that needed to wait until after the switch. */
Py_CLEAR(ts->st.del_post_switch);

This is code that does cleanup after tasklet switch, such as releasing the last remaining reference of the previous tasklet.

So, the bug was clear then. It was twofold:

  1. A Py_CLEAR() after switching was not careful enough to store the current tasklet’s “tempval” out of harms way of any side-effects a Py_DECREF() might cause, and
  2. Stack slicing itself, when it happened, clobbered the current tasklet’s “tempval”

The bug was subsequently fixed by repairing stack spilling and spiriting “tempval” away during the Py_CLEAR() call.

Post mortem

The inter-thread communication turned out to be a red herring. The problem was caused by an unfortunate juxtaposition of channel communication, tasklet deletion, and stack spilling.
But why had we not seen this before? I think it is largely due to the fact that stack spilling only rarely comes into play on regular platforms. On the PS3, we deliberately set the threshold low to conserve memory space. This is also not the first stack-spilling related bug we have seen on the PS3, but the first one for two years. Hopefully it will be the last.

Since this morning, the fix is in the stackless repository at http://hg.python.org/stackless

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