btrfs: introduce BTRFS_NESTING_COW for cow'ing blocks

When we COW a block we are holding a lock on the original block, and
then we lock the new COW block.  Because our lockdep maps are based on
root + level, this will make lockdep complain.  We need a way to
indicate a subclass for locking the COW'ed block, so plumb through our
btrfs_lock_nesting from btrfs_cow_block down to the btrfs_init_buffer,
and then introduce BTRFS_NESTING_COW to be used for cow'ing blocks.

The reason I've added all this extra infrastructure is because there
will be need of different nesting classes in follow up patches.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/fs/btrfs/locking.h b/fs/btrfs/locking.h
index 5fabda8f0a80..8b47ba34fb03 100644
--- a/fs/btrfs/locking.h
+++ b/fs/btrfs/locking.h
@@ -25,6 +25,14 @@ enum btrfs_lock_nesting {
 	BTRFS_NESTING_NORMAL,
 
 	/*
+	 * When we COW a block we are holding the lock on the original block,
+	 * and since our lockdep maps are rootid+level, this confuses lockdep
+	 * when we lock the newly allocated COW'd block.  Handle this by having
+	 * a subclass for COW'ed blocks so that lockdep doesn't complain.
+	 */
+	BTRFS_NESTING_COW,
+
+	/*
 	 * 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