How does RCU work?

[TAKADA Yoshihito <takada@mbf.nifty.com>]

Hi.
I cannot understand mechanism. Somebody tell me it please.

In x86 kernel, CONFIG_PREEMPT_RCU=n (CONFIG_CLASSIC_RCU=y). rcu_read_lock() makes
just preempt_disable().
call_cru()/synchronize_rcu() running as a soft-IRQ.
It's current CPU does not refer to protected data that when call_rcu() run. 
Because soft-IRQ handler ran, it means current CPU is preempive. That is the CPU
called rcu_read_unlock().

Well, current CPU is unlocked, but How does kernel know it other CPUs unlocked?

call_ruc() calls force_quiescent_state() to reschedule other CPUs. 
force_quiescent_state() calls smp_send_reschedule() to sends request to reshecule.
It's just require to reschedule via inter Processor Interrupt. But call_rcu() does
not wait for target CPUs handle IPI. Or, Do CPUs have it handled IPI request when
return from smp_send_reschedule()?