reproduce (this is a W=1 build):
        git remote add dwmw2 git://git.infradead.org/users/dwmw2/linux
        git fetch --no-tags dwmw2 pfncache-rcu-debug
        git checkout 024c810fe3ba3772608c193103449d9f1f7f5c37

        # 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