From cf6f34aa3ada0be8c5f90fe93f48a75fea082c51 Mon Sep 17 00:00:00 2001
From: Josef Bacik <josef@toxicpanda.com>
Date: Thu, 20 Aug 2020 11:46:07 -0400
Subject: [PATCH] btrfs: introduce BTRFS_NESTING_NEW_ROOT for adding new roots
The way we add new roots is confusing from a locking perspective for
lockdep. We generally have the rule that we lock things in order from
highest level to lowest, but in the case of adding a new level to the
tree we actually allocate a new block for the root, which makes the
locking go in reverse. A similar issue exists for snapshotting, we cow
the original root for the root of a new tree, however they're at the
same level. Address this by using BTRFS_NESTING_NEW_ROOT for these
operations.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
---
fs/btrfs/ctree.c | 5 +++--
fs/btrfs/locking.h | 9 +++++++++
2 files changed, 12 insertions(+), 2 deletions(-)
diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 884c8b5a0e626..d61ea238ad8a1 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -198,7 +198,8 @@ int btrfs_copy_root(struct btrfs_trans_handle *trans,
btrfs_node_key(buf, &disk_key, 0);
cow = btrfs_alloc_tree_block(trans, root, 0, new_root_objectid,
- &disk_key, level, buf->start, 0, BTRFS_NESTING_NORMAL);
+ &disk_key, level, buf->start, 0,
+ BTRFS_NESTING_NEW_ROOT);
if (IS_ERR(cow))
return PTR_ERR(cow);
@@ -3407,7 +3408,7 @@ static noinline int insert_new_root(struct btrfs_trans_handle *trans,
c = alloc_tree_block_no_bg_flush(trans, root, 0, &lower_key, level,
root->node->start, 0,
- BTRFS_NESTING_NORMAL);
+ BTRFS_NESTING_NEW_ROOT);
if (IS_ERR(c))
return PTR_ERR(c);
diff --git a/fs/btrfs/locking.h b/fs/btrfs/locking.h
index a6b59808e0460..3ea81ed3320b5 100644
--- a/fs/btrfs/locking.h
+++ b/fs/btrfs/locking.h
@@ -61,6 +61,15 @@ enum btrfs_lock_nesting {
*/
BTRFS_NESTING_SPLIT,
+ /*
+ * When promoting a new block to a root we need to have a special
+ * subclass so we don't confuse lockdep, as it will appear that we are
+ * locking a higher level node before a lower level one. Copying also
+ * has this problem as it appears we're locking the same block again
+ * when we make a snapshot of an existing root.
+ */
+ BTRFS_NESTING_NEW_ROOT,
+
/*
* We are limited to MAX_LOCKDEP_SUBLCLASSES number of subclasses, so
* add this in here and add a static_assert to keep us from going over