Due to the asynchronous nature of the dlm api, when we request a pending
locking request to be canceled with dlm_unlock(DLM_LKF_CANCEL), the
locking request will either complete before it could be canceled, or the
cancellation will succeed.  In either case, gdlm_ast will be called once
and the status will indicate the outcome of the locking request, with
-DLM_ECANCEL indicating a canceled request.

Inside dlm, when a locking request completes before its cancel request
could be processed, gdlm_ast will be called, but the lock will still be
considered busy until a DLM_MSG_CANCEL_REPLY message completes the
cancel request.  During that time, successive dlm_lock() or dlm_unlock()
requests for that lock will return -EBUSY.  In other words, waiting for
the gdlm_ast call before issuing the next locking request is not enough.
There is no way of waiting for a cancel request to actually complete,
either.

We rarely cancel locking requests, but when we do, we don't know when
the next locking request for that lock will occur.  This means that any
dlm_lock() or dlm_unlock() call can potentially return -EBUSY.  When
that happens, this patch simply repeats the request after a short pause.

This workaround could be improved upon by tracking for which dlm locks
cancel requests have been issued, but that isn't strictly necessary and
it would complicate the code.  We haven't seen -EBUSY errors from dlm
without cancel requests.
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>