reproduce (this is a W=1 build):
        git remote add hch-xfs git://git.infradead.org/users/hch/xfs
        git fetch --no-tags hch-xfs xfs-btree-cleanups
        git checkout 40f7bff05193f552b08efc497c5e81f9112348e5
        # 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/xfs/