Using CONFIG_PREEMPT_RCU and CONFIG_NO_IDLE_HZ on s390 my system always
gets stuck when running with more than one cpu.
When booting with four cpus I get all four cpus caught withing cpu_idle
and not advancing anymore. However there is the init process which is
waitung for synchronize_rcu() to complete (lcrash output):

STACK TRACE FOR TASK: 0xf84d968 (swapper)

 STACK:
 0 schedule+842 [0x36c956]
 1 schedule_timeout+172 [0x36d0e4]
 2 wait_for_common+204 [0x36c398]
 3 synchronize_rcu+76 [0x567bc]
 4 netlink_change_ngroups+150 [0x2b4302]
 5 genl_register_mc_group+256 [0x2b6174]
 6 genl_init+188 [0x534e44]
 7 kernel_init+444 [0x518334]
 8 kernel_thread_starter+6 [0x192a6]

If I change the code so that timer ticks won't be disabled everything
runs fine. So my guess is that rcu_needs_cpu() doesn't do the right
thing for the rcu preemptible case.

Kernel version is git head of today.

Any ideas?
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