When a group lock with GID=0 is released (put_grouplock is called), an assertion in cl_put_grouplock is hit.

There are two assertions at the start of cl_put_grouplock:
LASSERT(cg->cg_env);
LASSERT(cg->cg_gid);

cg_env is a pointer, so this assertion is valid, but cg_gid is just an integer so this is not a valid assertion. There are no other checks to make sure the GID is not zero, so a zero a GID is otherwise valid.

Since there doesn't appear to be any reason a '0' GID shouldn't be allowed, the solution is to remove this assertion. I'll make a patch shortly.