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[tip:smp/hotplug] sched/hotplug: Move sync_rcu to be with set_cpu_active(false)

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  [tip:smp/hotplug] sched/hotplug: Move sync_rcu to be with  set_cpu_active(false) tip-bot for Peter Zijlstra <tipbot@zytor.com> - 2016-05-05 13:30 +0200

#1395050 — [tip:smp/hotplug] sched/hotplug: Move sync_rcu to be with set_cpu_active(false)

Commit-ID:  c080b5a62379f0d26a5f3bc3eb80c93fdc888be4
Gitweb:     http://git.kernel.org/tip/c080b5a62379f0d26a5f3bc3eb80c93fdc888be4
Author:     Peter Zijlstra <peterz@infradead.org>
AuthorDate: Thu, 10 Mar 2016 12:54:14 +0100
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Thu, 5 May 2016 13:17:53 +0200

sched/hotplug: Move sync_rcu to be with set_cpu_active(false)

The sync_rcu stuff is specificically for clearing bits in the active
mask, such that everybody will observe the bit cleared and will not
consider the cleared CPU for load-balancing etc.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20160310120025.169219710@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
 kernel/cpu.c        | 15 ---------------
 kernel/sched/core.c | 14 ++++++++++++++
 2 files changed, 14 insertions(+), 15 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 15402b7..c134a35 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -703,21 +703,6 @@ static int takedown_cpu(unsigned int cpu)
 	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
 	int err;
 
-	/*
-	 * By now we've cleared cpu_active_mask, wait for all preempt-disabled
-	 * and RCU users of this state to go away such that all new such users
-	 * will observe it.
-	 *
-	 * For CONFIG_PREEMPT we have preemptible RCU and its sync_rcu() might
-	 * not imply sync_sched(), so wait for both.
-	 *
-	 * Do sync before park smpboot threads to take care the rcu boost case.
-	 */
-	if (IS_ENABLED(CONFIG_PREEMPT))
-		synchronize_rcu_mult(call_rcu, call_rcu_sched);
-	else
-		synchronize_rcu();
-
 	/* Park the smpboot threads */
 	kthread_park(per_cpu_ptr(&cpuhp_state, cpu)->thread);
 	smpboot_park_threads(cpu);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 73bcd93..0a31078 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7112,6 +7112,20 @@ int sched_cpu_deactivate(unsigned int cpu)
 	int ret;
 
 	set_cpu_active(cpu, false);
+	/*
+	 * We've cleared cpu_active_mask, wait for all preempt-disabled and RCU
+	 * users of this state to go away such that all new such users will
+	 * observe it.
+	 *
+	 * For CONFIG_PREEMPT we have preemptible RCU and its sync_rcu() might
+	 * not imply sync_sched(), so wait for both.
+	 *
+	 * Do sync before park smpboot threads to take care the rcu boost case.
+	 */
+	if (IS_ENABLED(CONFIG_PREEMPT))
+		synchronize_rcu_mult(call_rcu, call_rcu_sched);
+	else
+		synchronize_rcu();
 
 	if (!sched_smp_initialized)
 		return 0;

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