[rcu:rcu/next 91/91] ftrace.c:undefined reference to `synchronize_rcu_tasks'

[kbuild test robot]

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git rcu/next
head:   f1b89dcf23ef0999fae8bbc34cc9eb3fe00bc677
commit: f1b89dcf23ef0999fae8bbc34cc9eb3fe00bc677 [91/91] rcu: Remove Tasks RCU
config: x86_64-acpi-redef (attached as .config)
compiler: gcc-6 (Debian 6.2.0-3) 6.2.0 20160901
reproduce:
        git checkout f1b89dcf23ef0999fae8bbc34cc9eb3fe00bc677
        # save the attached .config to linux build tree
        make ARCH=x86_64 

All errors (new ones prefixed by >>):

   kernel/built-in.o: In function `ftrace_shutdown':
>> ftrace.c:(.text+0x9142d): undefined reference to `synchronize_rcu_tasks'

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