Btrfs: Don't loop forever on metadata IO failures

When a btrfs metadata read fails, the first thing we try to do is find
a good copy on another mirror of the block.  If this fails, read_tree_block()
ends up returning a buffer that isn't up to date.

The btrfs btree reading code was reworked to drop locks and repeat
the search when IO was done, but the changes didn't add a check for failed
reads.  The end result was looping forever on buffers that were never
going to become up to date.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.c

@@ -1469,6 +1469,7 @@ read_block_for_search(struct btrfs_trans_handle *trans,
 	u32 blocksize;
 	struct extent_buffer *b = *eb_ret;
 	struct extent_buffer *tmp;
+	int ret;
 
 	blocknr = btrfs_node_blockptr(b, slot);
 	gen = btrfs_node_ptr_generation(b, slot);
@@ -1476,6 +1477,10 @@ read_block_for_search(struct btrfs_trans_handle *trans,
 
 	tmp = btrfs_find_tree_block(root, blocknr, blocksize);
 	if (tmp && btrfs_buffer_uptodate(tmp, gen)) {
+		/*
+		 * we found an up to date block without sleeping, return
+		 * right away
+		 */
 		*eb_ret = tmp;
 		return 0;
 	}
@@ -1483,7 +1488,9 @@ read_block_for_search(struct btrfs_trans_handle *trans,
 	/*
 	 * reduce lock contention at high levels
 	 * of the btree by dropping locks before
-	 * we read.
+	 * we read.  Don't release the lock on the current
+	 * level because we need to walk this node to figure
+	 * out which blocks to read.
 	 */
 	btrfs_unlock_up_safe(p, level + 1);
 	btrfs_set_path_blocking(p);
@@ -1494,10 +1501,21 @@ read_block_for_search(struct btrfs_trans_handle *trans,
 		reada_for_search(root, p, level, slot, key->objectid);
 
 	btrfs_release_path(NULL, p);
+
+	ret = -EAGAIN;
 	tmp = read_tree_block(root, blocknr, blocksize, gen);
-	if (tmp)
+	if (tmp) {
+		/*
+		 * If the read above didn't mark this buffer up to date,
+		 * it will never end up being up to date.  Set ret to EIO now
+		 * and give up so that our caller doesn't loop forever
+		 * on our EAGAINs.
+		 */
+		if (!btrfs_buffer_uptodate(tmp, 0))
+			ret = -EIO;
 		free_extent_buffer(tmp);
-	return -EAGAIN;
+	}
+	return ret;
 }
 
 /*
@@ -1696,6 +1714,9 @@ cow_done:
 			if (ret == -EAGAIN)
 				goto again;
 
+			if (ret == -EIO)
+				goto done;
+
 			if (!p->skip_locking) {
 				int lret;
 
@@ -1738,6 +1759,8 @@ done:
 	 */
 	if (!p->leave_spinning)
 		btrfs_set_path_blocking(p);
+	if (ret < 0)
+		btrfs_release_path(root, p);
 	return ret;
 }
 
@@ -4212,6 +4235,11 @@ again:
 		if (ret == -EAGAIN)
 			goto again;
 
+		if (ret < 0) {
+			btrfs_release_path(root, path);
+			goto done;
+		}
+
 		if (!path->skip_locking) {
 			ret = btrfs_try_spin_lock(next);
 			if (!ret) {
@@ -4246,6 +4274,11 @@ again:
 		if (ret == -EAGAIN)
 			goto again;
 
+		if (ret < 0) {
+			btrfs_release_path(root, path);
+			goto done;
+		}
+
 		if (!path->skip_locking) {
 			btrfs_assert_tree_locked(path->nodes[level]);
 			ret = btrfs_try_spin_lock(next);