reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/bigeasy/staging.git/commit/?id=e5c4029a61dc897cc0ab99e2647f0e796bdde2bf
        git remote add bigeasy-staging https://git.kernel.org/pub/scm/linux/kernel/git/bigeasy/staging.git
        git fetch --no-tags bigeasy-staging ktimers_v2.5
        git checkout e5c4029a61dc897cc0ab99e2647f0e796bdde2bf
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 olddefconfig
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash kernel/rcu/