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>

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index ad61da79b311e131c159723d2003a833a4325dd6..d01e26df55a1375904b76532df4153834940c2f4 100644 (file)
--- a/kernel/rcutree_plugin.h
+++ b/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. */