[tip:smp/hotplug] cpu/hotplug: Plug death reporting race

[tip-bot for Thomas Gleixner <tipbot@zytor.com>]

Commit-ID:  71f87b2fc64c2e9b6d53cb817f28711b959d3dfe
Gitweb:     http://git.kernel.org/tip/71f87b2fc64c2e9b6d53cb817f28711b959d3dfe
Author:     Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Thu, 3 Mar 2016 10:52:10 +0100
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Thu, 3 Mar 2016 10:52:10 +0100

cpu/hotplug: Plug death reporting race

Paul noticed that the conversion of the death reporting introduced a race
where the outgoing cpu might be delayed after waking the controll processor,
so it might not be able to call rcu_report_dead() before being physically
removed, leading to RCU stalls.

We cant call complete after rcu_report_dead(), so instead of going back to
busy polling, simply issue a function call to do the completion.

Fixes: 27d50c7eeb0f "rcu: Make CPU_DYING_IDLE an explicit call"
Reported-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20160302201127.GA23440@linux.vnet.ibm.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra <peterz@infradead.org>
---
 kernel/cpu.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index ff8059b..93e9d89 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -755,14 +755,26 @@ static int notify_dead(unsigned int cpu)
 	return 0;
 }
 
+static void cpuhp_complete_idle_dead(void *arg)
+{
+	struct cpuhp_cpu_state *st = arg;
+
+	complete(&st->done);
+}
+
 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);
 	rcu_report_dead(smp_processor_id());
+	st->state = CPUHP_AP_IDLE_DEAD;
+	/*
+	 * We cannot call complete after rcu_report_dead() so we delegate it
+	 * to an online cpu.
+	 */
+	smp_call_function_single(cpumask_first(cpu_online_mask),
+				 cpuhp_complete_idle_dead, st, 0);
 }
 
 #else