rcu: make treercu safe for suspend and resume

Impact: fix kernel warnings [and potential crash] during suspend+resume

Kudos to both Dhaval Giani and Jens Axboe for finding a bug in treercu
that causes warnings after suspend-resume cycles in Dhaval's case and
during stress tests in Jens's case.  It would also probably cause failures
if heavily stressed.  The solution, ironically enough, is to revert to
rcupreempt's code for initializing the dynticks state.  And the patch
even results in smaller code -- so what was I thinking???

This is 2.6.29 material, given that people really do suspend and resume
Linux these days.  ;-)

Reported-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Reported-by: Jens Axboe <jens.axboe@oracle.com>
Tested-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Tested-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/rcutree.c

@@ -79,7 +79,10 @@ struct rcu_state rcu_bh_state = RCU_STATE_INITIALIZER(rcu_bh_state);
 DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);
 
 #ifdef CONFIG_NO_HZ
-DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks);
+DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
+	.dynticks_nesting = 1,
+	.dynticks = 1,
+};
 #endif /* #ifdef CONFIG_NO_HZ */
 
 static int blimit = 10;		/* Maximum callbacks per softirq. */
@@ -1380,13 +1383,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
 
 static void __cpuinit rcu_online_cpu(int cpu)
 {
-#ifdef CONFIG_NO_HZ
-	struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
-
-	rdtp->dynticks_nesting = 1;
-	rdtp->dynticks |= 1; 	/* need consecutive #s even for hotplug. */
-	rdtp->dynticks_nmi = (rdtp->dynticks_nmi + 1) & ~0x1;
-#endif /* #ifdef CONFIG_NO_HZ */
 	rcu_init_percpu_data(cpu, &rcu_state);
 	rcu_init_percpu_data(cpu, &rcu_bh_state);
 	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);