Commit 233e7f26 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Ingo Molnar
Browse files

stop_machine: Ensure that a queued callback will be called before cpu_stop_park() 



cpu_stop_queue_work() checks stopper->enabled before it queues the
work, but ->enabled == T can only guarantee cpu_stop_signal_done()
if we race with cpu_down().

This is not enough for stop_two_cpus() or stop_machine(), they will
deadlock if multi_cpu_stop() won't be called by one of the target
CPU's. stop_machine/stop_cpus are fine, they rely on stop_cpus_mutex.
But stop_two_cpus() has to check cpu_active() to avoid the same race
with hotplug, and this check is very unobvious and probably not even
correct if we race with cpu_up().

Change cpu_down() pass to clear ->enabled before cpu_stopper_thread()
flushes the pending ->works and returns with KTHREAD_SHOULD_PARK set.

Note also that smpboot_thread_call() calls cpu_stop_unpark() which
sets enabled == T at CPU_ONLINE stage, so this CPU can't go away until
cpu_stopper_thread() is called at least once. This all means that if
cpu_stop_queue_work() succeeds, we know that work->fn() will be called.

Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: heiko.carstens@de.ibm.com
Link: http://lkml.kernel.org/r/20151008145131.GA18139@redhat.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 6af597de

include/linux/stop_machine.h

+1 −0
Original line number Original line Diff line number Diff line
@@ -33,6 +33,7 @@ void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg, 
			 struct cpu_stop_work *work_buf);

			 struct cpu_stop_work *work_buf);

int stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg);

int stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg);

int try_stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg);

int try_stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg);

void stop_machine_park(int cpu);





#else	/* CONFIG_SMP */

#else	/* CONFIG_SMP */

kernel/cpu.c

+1 −1
Original line number Original line Diff line number Diff line
@@ -344,7 +344,7 @@ static int take_cpu_down(void *_param) 
	/* Give up timekeeping duties */

	/* Give up timekeeping duties */

	tick_handover_do_timer();

	tick_handover_do_timer();

	/* Park the stopper thread */

	/* Park the stopper thread */

	kthread_park(current);

	stop_machine_park((long)param->hcpu);

	return 0;

	return 0;

}

}

kernel/stop_machine.c

+13 −10
Original line number Original line Diff line number Diff line
@@ -452,6 +452,18 @@ repeat: 
	}

	}

}

}





void stop_machine_park(int cpu)

{

	struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);

	/*

	 * Lockless. cpu_stopper_thread() will take stopper->lock and flush

	 * the pending works before it parks, until then it is fine to queue

	 * the new works.

	 */

	stopper->enabled = false;

	kthread_park(stopper->thread);

}



extern void sched_set_stop_task(int cpu, struct task_struct *stop);

extern void sched_set_stop_task(int cpu, struct task_struct *stop);





static void cpu_stop_create(unsigned int cpu)

static void cpu_stop_create(unsigned int cpu)
@@ -462,17 +474,8 @@ static void cpu_stop_create(unsigned int cpu) 
static void cpu_stop_park(unsigned int cpu)

static void cpu_stop_park(unsigned int cpu)

{

{

	struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);

	struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);

	struct cpu_stop_work *work, *tmp;

	unsigned long flags;





	/* drain remaining works */

	WARN_ON(!list_empty(&stopper->works));

	spin_lock_irqsave(&stopper->lock, flags);

	list_for_each_entry_safe(work, tmp, &stopper->works, list) {

		list_del_init(&work->list);

		cpu_stop_signal_done(work->done, false);

	}

	stopper->enabled = false;

	spin_unlock_irqrestore(&stopper->lock, flags);

}

}





static void cpu_stop_unpark(unsigned int cpu)

static void cpu_stop_unpark(unsigned int cpu)

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