[PATCH 07/12] btrfs: introduce BTRFS_NESTING_SPLIT for split blocks

[Josef Bacik <josef@toxicpanda.com>]

If we are splitting a leaf/node, we could do something like the
following

lock(leaf)  BTRFS_NESTING_NORMAL
  lock(left) BTRFS_NESTING_LEFT + BTRFS_NESTING_COW
    push from leaf -> left
      reset path to point to left
        split left
          allocate new block, lock block BTRFS_NESTING_SPLIT

at the new block point we need to have a different nesting level,
because we have already used either BTRFS_NESTING_LEFT or
BTRFS_NESTING_RIGHT when pushing items from the original leaf into the
adjacent leaves.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/ctree.c   | 4 ++--
 fs/btrfs/locking.h | 9 +++++++++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index ae5e3f1056d8..4b62acac31b6 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -3473,7 +3473,7 @@ static noinline int split_node(struct btrfs_trans_handle *trans,
 	btrfs_node_key(c, &disk_key, mid);
 
 	split = alloc_tree_block_no_bg_flush(trans, root, 0, &disk_key, level,
-					     c->start, 0, BTRFS_NESTING_NORMAL);
+					     c->start, 0, BTRFS_NESTING_SPLIT);
 	if (IS_ERR(split))
 		return PTR_ERR(split);
 
@@ -4250,7 +4250,7 @@ static noinline int split_leaf(struct btrfs_trans_handle *trans,
 		btrfs_item_key(l, &disk_key, mid);
 
 	right = alloc_tree_block_no_bg_flush(trans, root, 0, &disk_key, 0,
-					     l->start, 0, BTRFS_NESTING_NORMAL);
+					     l->start, 0, BTRFS_NESTING_SPLIT);
 	if (IS_ERR(right))
 		return PTR_ERR(right);
 
diff --git a/fs/btrfs/locking.h b/fs/btrfs/locking.h
index a88c6f59f616..3b5a948bf74f 100644
--- a/fs/btrfs/locking.h
+++ b/fs/btrfs/locking.h
@@ -47,6 +47,15 @@ enum btrfs_lock_nesting {
 	BTRFS_NESTING_LEFT_COW,
 	BTRFS_NESTING_RIGHT_COW,
 
+	/*
+	 * When splitting we may push nodes to the left or right, but still use
+	 * the subsequent nodes in our path, keeping our locks on those adjacent
+	 * blocks.  Thus when we go to allocate a new split block we've already
+	 * used up all of our available subclasses, so this subclass exists to
+	 * handle this case where we need to allocate a new split block.
+	 */
+	BTRFS_NESTING_SPLIT,
+
 	/*
 	 * 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
-- 
2.24.1