hrtimers: allow the hot-unplugging of all cpus

Impact: fix CPU hotplug hang on Power6 testbox

On architectures that support offlining all cpus (at least powerpc/pseries),
hot-unpluging the tick_do_timer_cpu can result in a system hang.

This comes from the fact that if the cpu going down happens to be the
cpu doing the tick, then as the tick_do_timer_cpu handover happens after the
cpu is dead (via the CPU_DEAD notification), we're left without ticks,
jiffies are frozen and any task relying on timers (msleep, ...) is stuck.
That's particularly the case for the cpu looping in __cpu_die() waiting
for the dying cpu to be dead.

This patch addresses this by having the tick_do_timer_cpu handover happen
earlier during the CPU_DYING notification. For this, a new clockevent
notification type is introduced (CLOCK_EVT_NOTIFY_CPU_DYING) which is triggered
in hrtimer_cpu_notify().

Signed-off-by: Sebastien Dugue <sebastien.dugue@bull.net>
Cc: <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

3 files changed, 24 insertions, 7 deletions

diff --git a/include/linux/clockchips.h b/include/linux/clockchips.h
index ed3a5d473e52..c6de413c5dd1 100644
--- a/include/linux/clockchips.h
+++ b/include/linux/clockchips.h
@@ -36,6 +36,7 @@ enum clock_event_nofitiers {
         CLOCK_EVT_NOTIFY_BROADCAST_EXIT,
         CLOCK_EVT_NOTIFY_SUSPEND,
         CLOCK_EVT_NOTIFY_RESUME,
+        CLOCK_EVT_NOTIFY_CPU_DYING,
         CLOCK_EVT_NOTIFY_CPU_DEAD,
 };
 
diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index c2a69b89ac61..61cb933395ba 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -1609,6 +1609,10 @@ static int __cpuinit hrtimer_cpu_notify(struct notifier_block *self,
                 break;
 
 #ifdef CONFIG_HOTPLUG_CPU
+        case CPU_DYING:
+        case CPU_DYING_FROZEN:
+                clockevents_notify(CLOCK_EVT_NOTIFY_CPU_DYING, &scpu);
+                break;
         case CPU_DEAD:
         case CPU_DEAD_FROZEN:
         {
diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index df12434b43ca..457d281258ee 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -274,6 +274,21 @@ out_bc:
 }
 
 /*
+ * Transfer the do_timer job away from a dying cpu.
+ *
+ * Called with interrupts disabled.
+ */
+static void tick_handover_do_timer(int *cpup)
+{
+        if (*cpup == tick_do_timer_cpu) {
+                int cpu = first_cpu(cpu_online_map);
+
+                tick_do_timer_cpu = (cpu != NR_CPUS) ? cpu :
+                        TICK_DO_TIMER_NONE;
+        }
+}
+
+/*
  * Shutdown an event device on a given cpu:
  *
  * This is called on a life CPU, when a CPU is dead. So we cannot
@@ -297,13 +312,6 @@ static void tick_shutdown(unsigned int *cpup)
                 clockevents_exchange_device(dev, NULL);
                 td->evtdev = NULL;
         }
-        /* Transfer the do_timer job away from this cpu */
-        if (*cpup == tick_do_timer_cpu) {
-                int cpu = first_cpu(cpu_online_map);
-
-                tick_do_timer_cpu = (cpu != NR_CPUS) ? cpu :
-                        TICK_DO_TIMER_NONE;
-        }
         spin_unlock_irqrestore(&tick_device_lock, flags);
 }
 
@@ -357,6 +365,10 @@ static int tick_notify(struct notifier_block *nb, unsigned long reason,
                 tick_broadcast_oneshot_control(reason);
                 break;
 
+        case CLOCK_EVT_NOTIFY_CPU_DYING:
+                tick_handover_do_timer(dev);
+                break;
+
         case CLOCK_EVT_NOTIFY_CPU_DEAD:
                 tick_shutdown_broadcast_oneshot(dev);
                 tick_shutdown_broadcast(dev);