reproduce (this is a W=1 build):
        # https://github.com/kdave/btrfs-devel/commit/b6077834544fe4ed3d1e87f25271442ca26a319a
        git remote add kdave-btrfs-devel https://github.com/kdave/btrfs-devel.git
        git fetch --no-tags kdave-btrfs-devel misc-next
        git checkout b6077834544fe4ed3d1e87f25271442ca26a319a
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash fs/btrfs/