Btrfs: Improve and cleanup locking done by walk_down_tree

While dropping snapshots, walk_down_tree does most of the work of checking
reference counts and limiting tree traversal to just the blocks that
we are freeing.

It dropped and held the allocation mutex in strange and confusing ways,
this commit changes it to only hold the mutex while actually freeing a block.

The rest of the checks around reference counts should be safe without the lock
because we only allow one process in btrfs_drop_snapshot at a time.  Other
processes dropping reference counts should not drop it to 1 because
their tree roots already have an extra ref on the block.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 5e627746c4e8..224da287b3ed 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -268,6 +268,12 @@ again:
                 }
                 set_page_extent_mapped(page);
 
+                /*
+                 * this makes sure page_mkwrite is called on the
+                 * page if it is dirtied again later
+                 */
+                clear_page_dirty_for_io(page);
+
                 set_extent_delalloc(io_tree, page_start,
                                     page_end, GFP_NOFS);