drm/ttm: Fix two race conditions + fix busy codepaths

This fixes a race pointed out by Dave Airlie where we don't take a buffer
object about to be destroyed off the LRU lists properly. It also fixes a rare
case where a buffer object could be destroyed in the middle of an
accelerated eviction.

The patch also adds a utility function that can be used to prematurely
release GPU memory space usage of an object waiting to be destroyed.
For example during eviction or swapout.

The above mentioned commit didn't queue the buffer on the delayed destroy
list under some rare circumstances. It also didn't completely honor the
remove_all parameter.

Fixes:
https://bugzilla.redhat.com/show_bug.cgi?id=615505
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=591061

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/ttm/ttm_bo.c

@@ -441,6 +441,43 @@ out_err:
 	return ret;
 }
 
+/**
+ * Call bo::reserved and with the lru lock held.
+ * Will release GPU memory type usage on destruction.
+ * This is the place to put in driver specific hooks.
+ * Will release the bo::reserved lock and the
+ * lru lock on exit.
+ */
+
+static void ttm_bo_cleanup_memtype_use(struct ttm_buffer_object *bo)
+{
+	struct ttm_bo_global *glob = bo->glob;
+
+	if (bo->ttm) {
+
+		/**
+		 * Release the lru_lock, since we don't want to have
+		 * an atomic requirement on ttm_tt[unbind|destroy].
+		 */
+
+		spin_unlock(&glob->lru_lock);
+		ttm_tt_unbind(bo->ttm);
+		ttm_tt_destroy(bo->ttm);
+		bo->ttm = NULL;
+		spin_lock(&glob->lru_lock);
+	}
+
+	if (bo->mem.mm_node) {
+		drm_mm_put_block(bo->mem.mm_node);
+		bo->mem.mm_node = NULL;
+	}
+
+	atomic_set(&bo->reserved, 0);
+	wake_up_all(&bo->event_queue);
+	spin_unlock(&glob->lru_lock);
+}
+
+
 /**
  * If bo idle, remove from delayed- and lru lists, and unref.
  * If not idle, and already on delayed list, do nothing.
@@ -456,6 +493,7 @@ static int ttm_bo_cleanup_refs(struct ttm_buffer_object *bo, bool remove_all)
 	int ret;
 
 	spin_lock(&bo->lock);
+retry:
 	(void) ttm_bo_wait(bo, false, false, !remove_all);
 
 	if (!bo->sync_obj) {
@@ -464,31 +502,52 @@ static int ttm_bo_cleanup_refs(struct ttm_buffer_object *bo, bool remove_all)
 		spin_unlock(&bo->lock);
 
 		spin_lock(&glob->lru_lock);
-		put_count = ttm_bo_del_from_lru(bo);
+		ret = ttm_bo_reserve_locked(bo, false, !remove_all, false, 0);
 
-		ret = ttm_bo_reserve_locked(bo, false, false, false, 0);
+		/**
-		BUG_ON(ret);
+		 * Someone else has the object reserved. Bail and retry.
-		if (bo->ttm)
+		 */
-			ttm_tt_unbind(bo->ttm);
+
+		if (unlikely(ret == -EBUSY)) {
+			spin_unlock(&glob->lru_lock);
+			spin_lock(&bo->lock);
+			goto requeue;
+		}
+
+		/**
+		 * We can re-check for sync object without taking
+		 * the bo::lock since setting the sync object requires
+		 * also bo::reserved. A busy object at this point may
+		 * be caused by another thread starting an accelerated
+		 * eviction.
+		 */
+
+		if (unlikely(bo->sync_obj)) {
+			atomic_set(&bo->reserved, 0);
+			wake_up_all(&bo->event_queue);
+			spin_unlock(&glob->lru_lock);
+			spin_lock(&bo->lock);
+			if (remove_all)
+				goto retry;
+			else
+				goto requeue;
+		}
+
+		put_count = ttm_bo_del_from_lru(bo);
 
 		if (!list_empty(&bo->ddestroy)) {
 			list_del_init(&bo->ddestroy);
 			++put_count;
 		}
-		if (bo->mem.mm_node) {
-			drm_mm_put_block(bo->mem.mm_node);
-			bo->mem.mm_node = NULL;
-		}
-		spin_unlock(&glob->lru_lock);
 
-		atomic_set(&bo->reserved, 0);
+		ttm_bo_cleanup_memtype_use(bo);
 
 		while (put_count--)
 			kref_put(&bo->list_kref, ttm_bo_ref_bug);
 
 		return 0;
 	}
-
+requeue:
 	spin_lock(&glob->lru_lock);
 	if (list_empty(&bo->ddestroy)) {
 		void *sync_obj = bo->sync_obj;

include/drm/ttm/ttm_bo_api.h

@@ -246,9 +246,11 @@ struct ttm_buffer_object {
 
 	atomic_t reserved;
 
-
 	/**
 	 * Members protected by the bo::lock
+	 * In addition, setting sync_obj to anything else
+	 * than NULL requires bo::reserved to be held. This allows for
+	 * checking NULL while reserved but not holding bo::lock.
 	 */
 
 	void *sync_obj_arg;