Re: [Libguestfs] [PATCH NOT WORKING nbdkit 0/3] python: Allow thread model to be set from Python plugins.
On Wed, Aug 05, 2020 at 04:40:26PM +0100, Richard W.M. Jones wrote:
Patch 2 certainly allows you to set the thread model. However patch
3
shows that if you set it to nbdkit.THREAD_MODEL_PARALLEL it will
crash.
If you look closely at the stack trace (attached below) you can see
that ignoring threads which are in parts of nbdkit unrelated to
Python:
Thread 4: In pread, waiting in time.sleep(). This thread has released
the GIL.
Has it - I'm not seeing any call to PyEval_SaveThread()/RestoreThread
which is needed to release the GIL when going from Python to C.
Thread 2: Started to process a pread call but didn't reach Python code yet.
Thread 1: In pread, segfaults when checking if pread() is defined
in the Python code.
My understanding is this should all be OK and there's no reason for
Python to crash here. I wonder if it's because we're calling "down"
into Python from C, rather than the usual way of calling from Python
into C.
I'm also not seeing any coodes to PyGILState_Ensure()/Release which is
needed call into Python from C.
Without these calls, random crashes are certainly expected.
What libvirt-python does is
eg
PyThreadState *_save = NULL;
if (PyEval_ThreadsInitialized())
_save = PyEval_SaveThread();
...call C functions...
if (PyEval_ThreadsInitialized())
PyEval_RestoreThread(_save);
and
PyGILState_STATE _save = PyGILState_UNLOCKED;
if (PyEval_ThreadsInitialized())
_save = PyGILState_Ensure();
... call Python functions...
if (PyEval_ThreadsInitialized())
PyGILState_Release(_save);
Regards,
Daniel
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