[rcu:dev 119/161] ERROR: "rcu_barrier" [fs/isofs/isofs.ko] undefined!

[kbuild test robot]

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git dev
head:   472755d5ff3ee21ad16e0a0a06e087b7569eaffe
commit: 16bf6b3b61864e444bae5e4a7cf121f777893a8c [119/161] rcu: Express Tiny RCU updates in terms of RCU rather than RCU-sched
config: um-x86_64_defconfig (attached as .config)
compiler: gcc-7 (Debian 7.3.0-16) 7.3.0
reproduce:
        git checkout 16bf6b3b61864e444bae5e4a7cf121f777893a8c
        # save the attached .config to linux build tree
        make ARCH=um SUBARCH=x86_64

All errors (new ones prefixed by >>):

>> ERROR: "rcu_barrier" [fs/isofs/isofs.ko] undefined!

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