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Commit 0b26351b authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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stop_machine, sched: Fix migrate_swap() vs. active_balance() deadlock 



Matt reported the following deadlock:

CPU0					CPU1

schedule(.prev=migrate/0)		<fault>
  pick_next_task()			  ...
    idle_balance()			    migrate_swap()
      active_balance()			      stop_two_cpus()
						spin_lock(stopper0->lock)
						spin_lock(stopper1->lock)
						ttwu(migrate/0)
						  smp_cond_load_acquire() -- waits for schedule()
        stop_one_cpu(1)
	  spin_lock(stopper1->lock) -- waits for stopper lock

Fix this deadlock by taking the wakeups out from under stopper->lock.
This allows the active_balance() to queue the stop work and finish the
context switch, which in turn allows the wakeup from migrate_swap() to
observe the context and complete the wakeup.

Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reported-by: default avatarMatt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: default avatarMatt Fleming <matt@codeblueprint.co.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180420095005.GH4064@hirez.programming.kicks-ass.net


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent f4ef6a43

kernel/stop_machine.c

+14 −5
Original line number Diff line number Diff line
@@ -21,6 +21,7 @@ 
#include <linux/smpboot.h>

#include <linux/atomic.h>

#include <linux/nmi.h>

#include <linux/sched/wake_q.h>



/*

 * Structure to determine completion condition and record errors.  May
@@ -65,27 +66,31 @@ static void cpu_stop_signal_done(struct cpu_stop_done *done) 
}



static void __cpu_stop_queue_work(struct cpu_stopper *stopper,

					struct cpu_stop_work *work)

					struct cpu_stop_work *work,

					struct wake_q_head *wakeq)

{

	list_add_tail(&work->list, &stopper->works);

	wake_up_process(stopper->thread);

	wake_q_add(wakeq, stopper->thread);

}



/* queue @work to @stopper.  if offline, @work is completed immediately */

static bool cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work)

{

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

	DEFINE_WAKE_Q(wakeq);

	unsigned long flags;

	bool enabled;



	spin_lock_irqsave(&stopper->lock, flags);

	enabled = stopper->enabled;

	if (enabled)

		__cpu_stop_queue_work(stopper, work);

		__cpu_stop_queue_work(stopper, work, &wakeq);

	else if (work->done)

		cpu_stop_signal_done(work->done);

	spin_unlock_irqrestore(&stopper->lock, flags);



	wake_up_q(&wakeq);



	return enabled;

}
@@ -229,6 +234,7 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1, 
{

	struct cpu_stopper *stopper1 = per_cpu_ptr(&cpu_stopper, cpu1);

	struct cpu_stopper *stopper2 = per_cpu_ptr(&cpu_stopper, cpu2);

	DEFINE_WAKE_Q(wakeq);

	int err;

retry:

	spin_lock_irq(&stopper1->lock);
@@ -252,8 +258,8 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1, 
			goto unlock;



	err = 0;

	__cpu_stop_queue_work(stopper1, work1);

	__cpu_stop_queue_work(stopper2, work2);

	__cpu_stop_queue_work(stopper1, work1, &wakeq);

	__cpu_stop_queue_work(stopper2, work2, &wakeq);

unlock:

	spin_unlock(&stopper2->lock);

	spin_unlock_irq(&stopper1->lock);
@@ -263,6 +269,9 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1, 
			cpu_relax();

		goto retry;

	}



	wake_up_q(&wakeq);



	return err;

}

/**