Btrfs: do not do fast caching if we are allocating blocks for tree_root

Since the fast caching uses normal tree locking, we can possibly deadlock if we
get to the caching via a btrfs_search_slot() on the tree_root.  So just check to
see if the root we are on is the tree root, and just don't do the fast caching.
Reported-by: Sage Weil <sage@newdream.net>
Signed-off-by: Josef Bacik <josef@redhat.com>

fs/btrfs/extent-tree.c

@@ -429,6 +429,7 @@ static int caching_kthread(void *data)
 
 static int cache_block_group(struct btrfs_block_group_cache *cache,
 			     struct btrfs_trans_handle *trans,
+			     struct btrfs_root *root,
 			     int load_cache_only)
 {
 	struct btrfs_fs_info *fs_info = cache->fs_info;
@@ -442,9 +443,12 @@ static int cache_block_group(struct btrfs_block_group_cache *cache,
 
 	/*
 	 * We can't do the read from on-disk cache during a commit since we need
-	 * to have the normal tree locking.
+	 * to have the normal tree locking.  Also if we are currently trying to
+	 * allocate blocks for the tree root we can't do the fast caching since
+	 * we likely hold important locks.
 	 */
-	if (!trans->transaction->in_commit) {
+	if (!trans->transaction->in_commit &&
+	    (root && root != root->fs_info->tree_root)) {
 		spin_lock(&cache->lock);
 		if (cache->cached != BTRFS_CACHE_NO) {
 			spin_unlock(&cache->lock);
@@ -4083,7 +4087,7 @@ static int update_block_group(struct btrfs_trans_handle *trans,
 		 * space back to the block group, otherwise we will leak space.
 		 */
 		if (!alloc && cache->cached == BTRFS_CACHE_NO)
-			cache_block_group(cache, trans, 1);
+			cache_block_group(cache, trans, NULL, 1);
 
 		byte_in_group = bytenr - cache->key.objectid;
 		WARN_ON(byte_in_group > cache->key.offset);
@@ -4937,7 +4941,8 @@ static noinline int find_free_extent(struct btrfs_trans_handle *trans,
 		if (unlikely(block_group->cached == BTRFS_CACHE_NO)) {
 			u64 free_percent;
 
-			ret = cache_block_group(block_group, trans, 1);
+			ret = cache_block_group(block_group, trans,
+						orig_root, 1);
 			if (block_group->cached == BTRFS_CACHE_FINISHED)
 				goto have_block_group;
 
@@ -4961,7 +4966,8 @@ static noinline int find_free_extent(struct btrfs_trans_handle *trans,
 			if (loop > LOOP_CACHING_NOWAIT ||
 			    (loop > LOOP_FIND_IDEAL &&
 			     atomic_read(&space_info->caching_threads) < 2)) {
-				ret = cache_block_group(block_group, trans, 0);
+				ret = cache_block_group(block_group, trans,
+							orig_root, 0);
 				BUG_ON(ret);
 			}
 			found_uncached_bg = true;
@@ -5518,7 +5524,7 @@ int btrfs_alloc_logged_file_extent(struct btrfs_trans_handle *trans,
 	u64 num_bytes = ins->offset;
 
 	block_group = btrfs_lookup_block_group(root->fs_info, ins->objectid);
-	cache_block_group(block_group, trans, 0);
+	cache_block_group(block_group, trans, NULL, 0);
 	caching_ctl = get_caching_control(block_group);
 
 	if (!caching_ctl) {