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btrfs: introduce BTRFS_NESTING_NEW_ROOT for adding new roots

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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>
This commit is contained in:
Josef Bacik 2020-08-20 11:46:07 -04:00 committed by David Sterba
parent 4dff97e690
commit cf6f34aa3a
2 changed files with 12 additions and 2 deletions
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5
fs/btrfs/ctree.c
View File

@ -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);

9
fs/btrfs/locking.h
View File

@ -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