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Date:   Fri, 14 Feb 2020 10:40:14 -0500
From:   Sasha Levin <>
Cc:     Peter Zijlstra <>,
        "Paul E. McKenney" <>, Tejun Heo <>,
        Sasha Levin <>
Subject: [PATCH AUTOSEL 5.5 022/542] cpu/hotplug, stop_machine: Fix stop_machine vs hotplug order

From: Peter Zijlstra <>

[ Upstream commit 45178ac0cea853fe0e405bf11e101bdebea57b15 ]

Paul reported a very sporadic, rcutorture induced, workqueue failure.
When the planets align, the workqueue rescuer's self-migrate fails and
then triggers a WARN for running a work on the wrong CPU.

Tejun then figured that set_cpus_allowed_ptr()'s stop_one_cpu() call
could be ignored! When stopper->enabled is false, stop_machine will
insta complete the work, without actually doing the work. Worse, it
will not WARN about this (we really should fix this).

It turns out there is a small window where a freshly online'ed CPU is
marked 'online' but doesn't yet have the stopper task running:

	BP				AP

	  __cpu_up(cpu, idle)	 -->	start_secondary()
	    wait_for_ap_thread() <--	  cpuhp_online_idle()
					  while (1)

					... available to run kthreads ...

	      stopper->enable = true;

Close this by moving the stop_machine_unpark() into
cpuhp_online_idle(), such that the stopper thread is ready before we
start the idle loop and schedule.

Reported-by: "Paul E. McKenney" <>
Debugged-by: Tejun Heo <>
Signed-off-by: Peter Zijlstra (Intel) <>
Tested-by: "Paul E. McKenney" <>
Signed-off-by: Sasha Levin <>
 kernel/cpu.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 4dc279ed3b2d7..9c706af713fbc 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -525,8 +525,7 @@ static int bringup_wait_for_ap(unsigned int cpu)
 	if (WARN_ON_ONCE((!cpu_online(cpu))))
 		return -ECANCELED;
-	/* Unpark the stopper thread and the hotplug thread of the target cpu */
-	stop_machine_unpark(cpu);
+	/* Unpark the hotplug thread of the target cpu */
@@ -1089,8 +1088,8 @@ void notify_cpu_starting(unsigned int cpu)
  * Called from the idle task. Wake up the controlling task which brings the
- * stopper and the hotplug thread of the upcoming CPU up and then delegates
- * the rest of the online bringup to the hotplug thread.
+ * hotplug thread of the upcoming CPU up and then delegates the rest of the
+ * online bringup to the hotplug thread.
 void cpuhp_online_idle(enum cpuhp_state state)
@@ -1100,6 +1099,12 @@ void cpuhp_online_idle(enum cpuhp_state state)
 	if (state != CPUHP_AP_ONLINE_IDLE)
+	/*
+	 * Unpart the stopper thread before we start the idle loop (and start
+	 * scheduling); this ensures the stopper task is always available.
+	 */
+	stop_machine_unpark(smp_processor_id());
 	st->state = CPUHP_AP_ONLINE_IDLE;
 	complete_ap_thread(st, true);

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