Commit 45178ac0 authored Dec 10, 2019 by Peter Zijlstra

cpu/hotplug, stop_machine: Fix stop_machine vs hotplug order



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

	bringup_cpu()
	  __cpu_up(cpu, idle)	 -->	start_secondary()
					...
					cpu_startup_entry()
	  bringup_wait_for_ap()
	    wait_for_ap_thread() <--	  cpuhp_online_idle()
					  while (1)
					    do_idle()

					... available to run kthreads ...

	    stop_machine_unpark()
	      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" <paulmck@kernel.org>
Debugged-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: "Paul E. McKenney" <paulmck@kernel.org>

parent cde65194

kernel/cpu.c

+9 −4

Original line number	Diff line number	Diff line
		@@ -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 */
		kthread_unpark(st->thread);

		/*
		@@ -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)
		return;

		/*
		* 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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