clockevents: Fix cpu_down() race for hrtimer based broadcasting

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:

  7cba160a ("powernv/cpuidle: Redesign idle states management")

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

  5d1638ac ("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/2015/2/16/213Suggested-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>

include/linux/tick.h

@@ -36,6 +36,12 @@ extern void tick_irq_enter(void);
 static inline void tick_irq_enter(void) { }
 #endif
 
+#if defined(CONFIG_GENERIC_CLOCKEVENTS_BROADCAST) && defined(CONFIG_TICK_ONESHOT)
+extern void hotplug_cpu__broadcast_tick_pull(int dead_cpu);
+#else
+static inline void hotplug_cpu__broadcast_tick_pull(int dead_cpu) { }
+#endif
+
 #ifdef CONFIG_NO_HZ_COMMON
 extern int tick_nohz_tick_stopped(void);
 extern void tick_nohz_idle_enter(void);

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"
@@ -411,6 +412,7 @@ 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);
 

kernel/time/tick-broadcast.c

@@ -680,14 +680,19 @@ static void broadcast_shutdown_local(struct clock_event_device *bc,
 	clockevents_set_state(dev, CLOCK_EVT_STATE_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 */
+	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);
 }
 
 /*
@@ -924,8 +929,6 @@ 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);
 }