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Example code:
import os
import sys
import threading
def run():
 sys.stderr.write("in parent thread\n")
threading.Thread(target=run).start()
pid = os.fork()
if pid:
 os.waitpid(pid, 0)
else:
 sys.stderr.write("in child\n")
To run: while python3 deadlock.py; do; done 
Note: does not reproduce if ran with `python -u` (unbuffered)

This is a duplicate of http://bugs.python.org/issue6721 

Serhiy, I don't think this is a duplicate. Odd that you closed this without any explanation.
This happens in a internal lock in cpython's runtime, while the other bug is about locks used in the logging module (which are very different).

> Maries Ionel Cristian added the comment:
>
> Serhiy, I don't think this is a duplicate. Odd that you closed this without any explanation.
>
> This happens in a internal lock in cpython's runtime, while the other bug is about locks used in the logging module (which are very different).
Yes, this is a duplicate. Whether the lock is an internal or a user-created one doesn't change anything, it's always the same problem of having a lock locked by another thread at the time of the fork().

Sorry. I thought issue6721 is more general, not just about the logging module.