[PATCH] clockevents: Fix cpu_down() race for hrtimer based broadcasting

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

commit 345527b1edce8df719e0884500c76832a18211c3 upstream

It was found when doing a hotplug stress test on POWER, that the
machine either hit softlockups or rcu_sched stall warnings.  The
issue was traced to commit:

  7cba160ad789 ("powernv/cpuidle: Redesign idle states management")

which exposed the cpu_down() race with hrtimer based broadcast mode:

  5d1638acb9f6 ("tick: Introduce hrtimer based broadcast")

The race is the following:

Assume CPU1 is the CPU which holds the hrtimer broadcasting duty
before it is taken down.

	CPU0					CPU1

	cpu_down()				take_cpu_down()
						disable_interrupts()

	cpu_die()

	while (CPU1 != CPU_DEAD) {
		msleep(100);
		switch_to_idle();
		stop_cpu_timer();
		schedule_broadcast();
	}

	tick_cleanup_cpu_dead()
		take_over_broadcast()

So after CPU1 disabled interrupts it cannot handle the broadcast
hrtimer anymore, so CPU0 will be stuck forever.

Fix this by explicitly taking over broadcast duty before cpu_die().

This is a temporary workaround. What we really want is a callback
in the clockevent device which allows us to do that from the dying
CPU by pushing the hrtimer onto a different cpu. That might involve
an IPI and is definitely more complex than this immediate fix.

Changelog was picked up from:

    https://lkml.org/lkml/2015/2/16/213

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Preeti U. Murthy <preeti@linux.vnet.ibm.com>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: mpe@ellerman.id.au
Cc: nicolas.pitre@linaro.org
Cc: peterz@infradead.org
Cc: rjw@rjwysocki.net
Fixes: http://linuxppc.10917.n7.nabble.com/offlining-cpus-breakage-td88619.html
Link: http://lkml.kernel.org/r/20150330092410.24979.59887.stgit@preeti.in.ibm.com
[ Merged it to the latest timer tree, renamed the callback, tidied up the changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---

Please apply this to 3.19 stable.

 kernel/cpu.c                 |    2 ++
 kernel/time/tick-broadcast.c |   19 +++++++++++--------
 2 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 5d22023..53bec17 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -20,6 +20,7 @@
 #include <linux/gfp.h>
 #include <linux/suspend.h>
 #include <linux/lockdep.h>
+#include <linux/tick.h>
 #include <trace/events/power.h>
 
 #include "smpboot.h"
@@ -421,6 +422,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
 	while (!idle_cpu(cpu))
 		cpu_relax();
 
+	hotplug_cpu__broadcast_tick_pull(cpu);
 	/* This actually kills the CPU. */
 	__cpu_die(cpu);
 
diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index 066f0ec..25fb004 100644
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c
@@ -669,14 +669,19 @@ static void broadcast_shutdown_local(struct clock_event_device *bc,
 	clockevents_set_mode(dev, CLOCK_EVT_MODE_SHUTDOWN);
 }
 
-static void broadcast_move_bc(int deadcpu)
+void hotplug_cpu__broadcast_tick_pull(int deadcpu)
 {
-	struct clock_event_device *bc = tick_broadcast_device.evtdev;
+	struct clock_event_device *bc;
+	unsigned long flags;
 
-	if (!bc || !broadcast_needs_cpu(bc, deadcpu))
-		return;
-	/* This moves the broadcast assignment to this cpu */
-	clockevents_program_event(bc, bc->next_event, 1);
+	raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
+	bc = tick_broadcast_device.evtdev;
+
+	if (bc && broadcast_needs_cpu(bc, deadcpu)) {
+		/* This moves the broadcast assignment to this CPU: */
+		clockevents_program_event(bc, bc->next_event, 1);
+	}
+	raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
 }
 
 /*
@@ -913,8 +918,6 @@ void tick_shutdown_broadcast_oneshot(unsigned int *cpup)
 	cpumask_clear_cpu(cpu, tick_broadcast_pending_mask);
 	cpumask_clear_cpu(cpu, tick_broadcast_force_mask);
 
-	broadcast_move_bc(cpu);
-
 	raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
 }