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I too have verified that the testprograms (with an
additional sleep) cause a deadlock, and that moving the
import statement out of os._execvp solves the problem.


However, there is a more generic problem. If fork() is
executed while any locks are held by other threads than the
one executing fork, it will deadlock the new process.

E.g, if you have a process with 2 threads. The first one is
doing work that requires locking. This could be imports, but
also anything else that uses other locks.

The second thread does a fork. If this is accidently done
while thread 1 held the lock, it will deadlock if the new
process needs the same lock.

Now, the most common use of fork is of course to do exec
right afterwards, in which case it doesn't really matter,
but it's not the only use of fork. 

See the rationalie in the manpage of pthread_atfork for
details.
(http://www.opengroup.org/onlinepubs/009695399/functions/pthread_atfork.html)