rcu: Ensure that RCU_FAST_NO_HZ timers expire on correct CPU

Timers are subject to migration, which can lead to the following
system-hang scenario when CONFIG_RCU_FAST_NO_HZ=y:

1.	CPU 0 executes synchronize_rcu(), which posts an RCU callback.

2.	CPU 0 then goes idle.  It cannot immediately invoke the callback,
	but there is nothing RCU needs from ti, so it enters dyntick-idle
	mode after posting a timer.

3.	The timer gets migrated to CPU 1.

4.	CPU 0 never wakes up, so the synchronize_rcu() never returns, so
	the system hangs.

This commit fixes this problem by using mod_timer_pinned(), as suggested
by Peter Zijlstra, to ensure that the timer is actually posted on the
running CPU.
Reported-by: Dipankar Sarma <dipankar@in.ibm.com>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcutree_plugin.h

@@ -2110,6 +2110,8 @@ static void rcu_cleanup_after_idle(int cpu)
  */
 static void rcu_prepare_for_idle(int cpu)
 {
+	struct timer_list *tp;
+
 	/*
 	 * If this is an idle re-entry, for example, due to use of
 	 * RCU_NONIDLE() or the new idle-loop tracing API within the idle
@@ -2121,9 +2123,10 @@ static void rcu_prepare_for_idle(int cpu)
 	if (!per_cpu(rcu_idle_first_pass, cpu) &&
 	    (per_cpu(rcu_nonlazy_posted, cpu) ==
 	     per_cpu(rcu_nonlazy_posted_snap, cpu))) {
-		if (rcu_cpu_has_callbacks(cpu))
-			mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
-				  per_cpu(rcu_idle_gp_timer_expires, cpu));
+		if (rcu_cpu_has_callbacks(cpu)) {
+			tp = &per_cpu(rcu_idle_gp_timer, cpu);
+			mod_timer_pinned(tp, per_cpu(rcu_idle_gp_timer_expires, cpu));
+		}
 		return;
 	}
 	per_cpu(rcu_idle_first_pass, cpu) = 0;
@@ -2167,8 +2170,8 @@ static void rcu_prepare_for_idle(int cpu)
 		else
 			per_cpu(rcu_idle_gp_timer_expires, cpu) =
 					   jiffies + RCU_IDLE_LAZY_GP_DELAY;
-		mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
-			  per_cpu(rcu_idle_gp_timer_expires, cpu));
+		tp = &per_cpu(rcu_idle_gp_timer, cpu);
+		mod_timer_pinned(tp, per_cpu(rcu_idle_gp_timer_expires, cpu));
 		per_cpu(rcu_nonlazy_posted_snap, cpu) =
 			per_cpu(rcu_nonlazy_posted, cpu);
 		return; /* Nothing more to do immediately. */