drm/ttm: cope with reserved buffers on swap list in ttm_bo_swapout, v2

Replace the while loop with a simple for each loop, and only run the
delayed destroy cleanup if we can reserve the buffer first.

No race occurs, since lru lock is never dropped any more. An empty list
and a list full of unreservable buffers both cause -EBUSY to be returned,
which is identical to the previous situation.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/ttm/ttm_bo.c

@@ -1813,41 +1813,25 @@ static int ttm_bo_swapout(struct ttm_mem_shrink *shrink)
 	uint32_t swap_placement = (TTM_PL_FLAG_CACHED | TTM_PL_FLAG_SYSTEM);
 
 	spin_lock(&glob->lru_lock);
-	while (ret == -EBUSY) {
-		if (unlikely(list_empty(&glob->swap_lru))) {
-			spin_unlock(&glob->lru_lock);
-			return -EBUSY;
-		}
-
-		bo = list_first_entry(&glob->swap_lru,
-				      struct ttm_buffer_object, swap);
-		kref_get(&bo->list_kref);
-
-		if (!list_empty(&bo->ddestroy)) {
-			ttm_bo_reserve_locked(bo, false, false, false, 0);
-			ttm_bo_cleanup_refs_and_unlock(bo, false, false);
+	list_for_each_entry(bo, &glob->swap_lru, swap) {
+		ret = ttm_bo_reserve_locked(bo, false, true, false, 0);
+		if (!ret)
+			break;
+	}
 
-			kref_put(&bo->list_kref, ttm_bo_release_list);
-			spin_lock(&glob->lru_lock);
-			continue;
-		}
+	if (ret) {
+		spin_unlock(&glob->lru_lock);
+		return ret;
+	}
 
-		/**
-		 * Reserve buffer. Since we unlock while sleeping, we need
-		 * to re-check that nobody removed us from the swap-list while
-		 * we slept.
-		 */
+	kref_get(&bo->list_kref);
 
-		ret = ttm_bo_reserve_locked(bo, false, true, false, 0);
-		if (unlikely(ret == -EBUSY)) {
-			spin_unlock(&glob->lru_lock);
-			ttm_bo_wait_unreserved(bo, false);
-			kref_put(&bo->list_kref, ttm_bo_release_list);
-			spin_lock(&glob->lru_lock);
-		}
+	if (!list_empty(&bo->ddestroy)) {
+		ret = ttm_bo_cleanup_refs_and_unlock(bo, false, false);
+		kref_put(&bo->list_kref, ttm_bo_release_list);
+		return ret;
 	}
 
-	BUG_ON(ret != 0);
 	put_count = ttm_bo_del_from_lru(bo);
 	spin_unlock(&glob->lru_lock);