How to release the GIL from C?

[Roger Binns]

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I am trying to release the GIL in a multi-threaded program (efforts
detailed below) without any success.

In the main thread during startup, I do the following:

  Py_InitializeEx(0); /* Python shouldn't handle signals */
  PyEval_InitThreads();

  /* release the GIL */
  ... various pieces of code tried here ...

Note that no other threads exist yet when the code above is run.

After startup, there are around 30 threads and my handler routines do
work in them.

  void handler(...)
  {
     PyGILState_STATE gilstate=PyGILState_Ensure();

     ... do the interesting stuff ...

     PyGILState_Release(gilstate);
  }

No matter what I try, the PyGILState_Ensure() hangs trying to acquire
the GIL:

#0  0x00002afd50ce243d in sem_wait () from /lib/libpthread.so.0
#1  0x00002afd5471557d in PyThread_acquire_lock () from
/usr/lib/libpython2.5.so.1.0
#2  0x00002afd546e44d4 in PyEval_RestoreThread () from
/usr/lib/libpython2.5.so.1.0
#3  0x00002afd5470bb8a in PyGILState_Ensure () from
/usr/lib/libpython2.5.so.1.0

Here are the various things I have tried to release the GIL (the
documentation says they all do, but that doesn't appear to be the case
:-(  Unfortunately the doc is all geared around saving the interpreter
state and then later restoring it.  I have nowhere to save it in the
main thread nor do I know when the main thread is having handlers
invoked in it again anyway in order to restore it.

1. (Based on Py_BEGIN_ALLOW_THREADS)
  PyThreadState_Swap(NULL);
  PyEval_ReleaseLock();

2. (Based on high level Py_BEGIN_ALLOW_THREADS)
  PyEval_SaveThread();

3. Random code I found on the web
  PyThreadState *tstate = PyThreadState_Get();
  PyThreadState_Clear(tstate);
  PyEval_ReleaseThread(tstate);

4. Something simple
  PyEval_ReleaseLock();

5. Other code I found on the web
  PyThreadState_Swap(NULL);

Does anyone know exactly which sequence of calls I should be making?

Thanks,

Roger

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