The issue is indeed UP vs SMP and it is totally random.

spin_is_locked() is a bad assertion because there is no correct answer on UP.
on UP spin_is_locked() has to return either one value or another, always.

This means that in my setup I am lucky enough to trigger the issue and your you
are lucky enough not to.

the patch in attachment removes the bogus calls to BUG_ON and according to David
(in CC and thanks for the long explanation on the problem) we can rely upon
things like lockdep to find problem that might be trying to catch.

Signed-off-by: Fabio M. Di Nitto <fabbione@ubuntu.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>