cpu/hotplug: Make wait for dead cpu completion based

Kill the busy spinning on the control side and just wait for the hotplugged
cpu to tell that it reached the dead state.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org
Cc: Rik van Riel <riel@redhat.com>
Cc: Rafael Wysocki <rafael.j.wysocki@intel.com>
Cc: "Srivatsa S. Bhat" <srivatsa@mit.edu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul Turner <pjt@google.com>
Link: http://lkml.kernel.org/r/20160226182341.776157858@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/cpu.c

diff --git a/kernel/cpu.c b/kernel/cpu.c
index f1f880f..0e8c07f 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -688,6 +688,7 @@
 
 static int takedown_cpu(unsigned int cpu)
 {
+	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
 	int err;
 
 	/*
@@ -733,10 +734,8 @@
 	 *
 	 * Wait for the stop thread to go away.
 	 */
-	while (!per_cpu(cpu_dead_idle, cpu))
-		cpu_relax();
-	smp_mb(); /* Read from cpu_dead_idle before __cpu_die(). */
-	per_cpu(cpu_dead_idle, cpu) = false;
+	wait_for_completion(&st->done);
+	BUG_ON(st->state != CPUHP_AP_IDLE_DEAD);
 
 	/* Interrupts are moved away from the dying cpu, reenable alloc/free */
 	irq_unlock_sparse();
@@ -756,6 +755,15 @@
 	return 0;
 }
 
+void cpuhp_report_idle_dead(void)
+{
+	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
+
+	BUG_ON(st->state != CPUHP_AP_OFFLINE);
+	st->state = CPUHP_AP_IDLE_DEAD;
+	complete(&st->done);
+}
+
 #else
 #define notify_down_prepare	NULL
 #define takedown_cpu		NULL