TINY_RCU's rcu_idle_enter_common() invokes rcu_sched_qs() in order
to inform the RCU core of the quiescent state implied by idle entry.
Of course, idle is also an extended quiescent state, so that the call
to rcu_sched_qs() speeds up RCU's invoking of any callbacks that might
be queued.  This speed-up is important when entering into dyntick-idle
mode -- if there are no further scheduling-clock interrupts, the callbacks
might never be invoked, which could result in a system hang.

However, processing callbacks does event tracing, which in turn
implies RCU read-side critical sections, which are illegal in extended
quiescent states.  This patch therefore moves the call to rcu_sched_qs()
so that it precedes the point at which we inform lockdep that RCU has
entered an extended quiescent state.
Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>