drm/ttm: Fix a theoretical race in ttm_bo_cleanup_refs()

In theory, that function could release the lru lock between
checking for bo on ddestroy list and a successful reserve if the bo
was already reserved, and the function was called with waiting reserves
allowed.
However, all current reservers of a bo on the ddestroy list would
atomically take the bo off the list after a successful reserve so this
race should not have been hit, so no need to backport for stable.

This patch also fixes a case found by Maarten Lankhorst where
ttm_mem_evict_first called with no_wait_gpu would incorrectly
spin waiting for bo idle if trying to evict a busy buffer that
also sits on the ddestroy list.
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/ttm/ttm_bo.c

@@ -580,6 +580,7 @@ static int ttm_bo_cleanup_refs(struct ttm_buffer_object *bo,
 	if (unlikely(ret != 0))
 		return ret;
 
+retry_reserve:
 	spin_lock(&glob->lru_lock);
 
 	if (unlikely(list_empty(&bo->ddestroy))) {
@@ -587,14 +588,20 @@ static int ttm_bo_cleanup_refs(struct ttm_buffer_object *bo,
 		return 0;
 	}
 
-	ret = ttm_bo_reserve_locked(bo, interruptible,
-				    no_wait_reserve, false, 0);
+	ret = ttm_bo_reserve_locked(bo, false, true, false, 0);
 
-	if (unlikely(ret != 0)) {
+	if (unlikely(ret == -EBUSY)) {
 		spin_unlock(&glob->lru_lock);
+		if (likely(!no_wait_reserve))
+			ret = ttm_bo_wait_unreserved(bo, interruptible);
+		if (unlikely(ret != 0))
 			return ret;
+
+		goto retry_reserve;
 	}
 
+	BUG_ON(ret != 0);
+
 	/**
 	 * We can re-check for sync object without taking
 	 * the bo::lock since setting the sync object requires
@@ -811,10 +818,7 @@ static int ttm_mem_evict_first(struct ttm_bo_device *bdev,
 					  no_wait_reserve, no_wait_gpu);
 		kref_put(&bo->list_kref, ttm_bo_release_list);
 
-		if (likely(ret == 0 || ret == -ERESTARTSYS))
 		return ret;
-
-		goto retry;
 	}
 
 	ret = ttm_bo_reserve_locked(bo, false, true, false, 0);