race condition in owl imports

[lukemetz]

Hello again,
I think I found a race condition in importing of owl.
I have a fairly simple test program that crashes maybe 1 in 3 times.

import owl
import owl.elewise as ele

c = owl.create_cpu_device()
owl.set_device(c)

x = owl.zeros((10, 10))
y = ele.relu(x)

The error received on a bad run looks something like:

○ → python tt.py
[22:55:26] /home/luke/Repos/minerva/minerva/system/minerva_system.cpp:86: dag engine enabled
[22:55:26] /home/luke/Repos/minerva/minerva/backend/dag/dag_scheduler.cpp:46: create new op node #1 on device #0
[22:55:26] /home/luke/Repos/minerva/minerva/backend/dag/dag_scheduler.cpp:149: node #1 running right after creation
[22:55:26] /home/luke/Repos/minerva/minerva/backend/dag/dag_scheduler.cpp:46: create new op node #3 on device #0
[22:55:26] /home/luke/Repos/minerva/minerva/backend/dag/dag_scheduler.cpp:176: dispatching node #1 to device #0
[22:55:26] /home/luke/Repos/minerva/minerva/device/device.cpp:95: CPU device #0 create output for task data #0
[22:55:26] /home/luke/Repos/minerva/minerva/device/data_store.cpp:18: create data #0 length 400
terminate called after throwing an instance of 'dmlc::Error'
  what():  [22:55:26] minerva/common/singleton.h:13: Check failed: data_ please initialize before use
^[[AAborted (core dumped)

Sadly I cannot get a legit stack trace as when I run it under gdb I get no failure.

I can throw a sleep of a bit just after the import and it seems to fix the problem.

This is under cpu, ubuntu 15.04 with the dag enabled.

Thanks!