Optimistically wait for the prior vma activity before taking the mutex to minimise the mutex hold time while unbinding. We will then verify the vma is idle with a second wait under the mutex to ensure it is safe to unbind. Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20200123224459.38128-2-chris@chris-wilson.co.uk