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>
diff --git a/fs/btrfs/locking.h b/fs/btrfs/locking.h
index a6b5980..3ea81ed 100644
--- a/fs/btrfs/locking.h
+++ b/fs/btrfs/locking.h
@@ -62,6 +62,15 @@ 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
* the limit. As of this writing we're limited to 8, and we're