reproduce:
        # apt-get install sparse
        # sparse version: v0.6.4-66-g0196afe1
        # https://git.kernel.org/pub/scm/linux/kernel/git/boqun/linux.git/commit/?id=809c307dd8a074d866ad97209209e7b8a4249861
        git remote add boqun https://git.kernel.org/pub/scm/linux/kernel/git/boqun/linux.git
        git fetch --no-tags boqun dev/hazptr
        git checkout 809c307dd8a074d866ad97209209e7b8a4249861
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=i386 olddefconfig
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=i386 SHELL=/bin/bash kernel/rcu/