drm/i915: Check activity on i915_vma after confirming pin_count==0

Only assert that the i915_vma is now idle if and only if no other pins
are present. If another user has the i915_vma pinned, they may submit
more work to the i915_vma skipping the vm->mutex used to serialise the
unbind. We need to wait again, if we want to continue and unbind this
vma.

However, if we own the i915_vma (we hold the vm->mutex for the unbind
and the pin_count is 0), we can assert that the vma remains idle as we
unbind.

Fixes: 2850748ef8 ("drm/i915: Pull i915_vma_pin under the vm->mutex")
Closes: https://gitlab.freedesktop.org/drm/intel/issues/530
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/20200123224459.38128-1-chris@chris-wilson.co.uk
(cherry picked from commit 60e94557ff)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_vma.c

@@ -1202,16 +1202,26 @@ int __i915_vma_unbind(struct i915_vma *vma)
 	if (ret)
 		return ret;
 
-	GEM_BUG_ON(i915_vma_is_active(vma));
 	if (i915_vma_is_pinned(vma)) {
 		vma_print_allocator(vma, "is pinned");
 		return -EAGAIN;
 	}
 
-	GEM_BUG_ON(i915_vma_is_active(vma));
+	/*
+	 * After confirming that no one else is pinning this vma, wait for
+	 * any laggards who may have crept in during the wait (through
+	 * a residual pin skipping the vm->mutex) to complete.
+	 */
+	ret = i915_vma_sync(vma);
+	if (ret)
+		return ret;
+
 	if (!drm_mm_node_allocated(&vma->node))
 		return 0;
 
+	GEM_BUG_ON(i915_vma_is_pinned(vma));
+	GEM_BUG_ON(i915_vma_is_active(vma));
+
 	if (i915_vma_is_map_and_fenceable(vma)) {
 		/*
 		 * Check that we have flushed all writes through the GGTT