Thanks David, there is something 'interesting' being observed here.
At a point:
Calling testandset in thread 1, m.locked is False
Calling testandset in thread 0, m.locked is False
Thread 0 locked
Resetting, trying again

Another place:
Calling testandset in thread 1, m.locked is False
Calling testandset in thread 0, m.locked is False
Thread 0 locked
Thread 1 locked
Hah, all these threads locked at the same time: [0, 1]

My doubts are still with threading, but am unable to derive anything.
Should someone more experienced look into? Or mind taking this for suggestions to c.l.p?