From: Josef Bacik <josef@toxicpanda.com>
Date: Fri, 12 Mar 2021 15:25:33 -0500
Subject: btrfs: do proper error handling in merge_reloc_roots
Git-commit: 24213fa46c7080c31d79aa2e3e2f0d9480cab700
Patch-mainline: v5.13-rc1
References: bsc#1187833

We have a BUG_ON() if we get an error back from btrfs_get_fs_root().
This honestly should never fail, as at this point we have a solid
coordination of fs root to reloc root, and these roots will all be in
memory.  But in the name of killing BUG_ON()'s remove these and handle
the error condition properly, ASSERT()'ing for developers.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Acked-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/relocation.c |   25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -2654,8 +2654,29 @@ again:
 
 		root = read_fs_root(fs_info, reloc_root->root_key.offset);
 		if (btrfs_root_refs(&reloc_root->root_item) > 0) {
-			BUG_ON(IS_ERR(root));
-			BUG_ON(root->reloc_root != reloc_root);
+			if (IS_ERR(root)) {
+				/*
+				 * For recovery we read the fs roots on mount,
+				 * and if we didn't find the root then we marked
+				 * the reloc root as a garbage root.  For normal
+				 * relocation obviously the root should exist in
+				 * memory.  However there's no reason we can't
+				 * handle the error properly here just in case.
+				 */
+				ASSERT(0);
+				ret = PTR_ERR(root);
+				goto out;
+			}
+			if (root->reloc_root != reloc_root) {
+				/*
+				 * This is actually impossible without something
+				 * going really wrong (like weird race condition
+				 * or cosmic rays).
+				 */
+				ASSERT(0);
+				ret = -EINVAL;
+				goto out;
+			}
 			ret = merge_reloc_root(rc, root);
 			if (ret) {
 				if (list_empty(&reloc_root->root_list))