Commit 6d337eab authored by Peter Zijlstra's avatar Peter Zijlstra
Browse files

sched: Fix migrate_disable() vs set_cpus_allowed_ptr() 



Concurrent migrate_disable() and set_cpus_allowed_ptr() has
interesting features. We rely on set_cpus_allowed_ptr() to not return
until the task runs inside the provided mask. This expectation is
exported to userspace.

This means that any set_cpus_allowed_ptr() caller must wait until
migrate_enable() allows migrations.

At the same time, we don't want migrate_enable() to schedule, due to
patterns like:

	preempt_disable();
	migrate_disable();
	...
	migrate_enable();
	preempt_enable();

And:

	raw_spin_lock(&B);
	spin_unlock(&A);

this means that when migrate_enable() must restore the affinity
mask, it cannot wait for completion thereof. Luck will have it that
that is exactly the case where there is a pending
set_cpus_allowed_ptr(), so let that provide storage for the async stop
machine.

Much thanks to Valentin who used TLA+ most effective and found lots of
'interesting' cases.

Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarValentin Schneider <valentin.schneider@arm.com>
Reviewed-by: default avatarDaniel Bristot de Oliveira <bristot@redhat.com>
Link: https://lkml.kernel.org/r/20201023102346.921768277@infradead.org
parent af449901

include/linux/sched.h

+1 −0
Original line number Diff line number Diff line
@@ -714,6 +714,7 @@ struct task_struct { 
	int				nr_cpus_allowed;

	const cpumask_t			*cpus_ptr;

	cpumask_t			cpus_mask;

	void				*migration_pending;

#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT)

	int				migration_disabled;

#endif

kernel/sched/core.c

+206 −30
Original line number Diff line number Diff line
@@ -1732,15 +1732,26 @@ void migrate_enable(void) 
{

	struct task_struct *p = current;



	if (--p->migration_disabled)

		return;



	barrier();



	if (p->cpus_ptr == &p->cpus_mask)

	if (p->migration_disabled > 1) {

		p->migration_disabled--;

		return;

	}



	/*

	 * Ensure stop_task runs either before or after this, and that

	 * __set_cpus_allowed_ptr(SCA_MIGRATE_ENABLE) doesn't schedule().

	 */

	preempt_disable();

	if (p->cpus_ptr != &p->cpus_mask)

		__set_cpus_allowed_ptr(p, &p->cpus_mask, SCA_MIGRATE_ENABLE);

	/*

	 * Mustn't clear migration_disabled() until cpus_ptr points back at the

	 * regular cpus_mask, otherwise things that race (eg.

	 * select_fallback_rq) get confused.

	 */

	barrier();

	p->migration_disabled = 0;

	preempt_enable();

}

EXPORT_SYMBOL_GPL(migrate_enable);
@@ -1807,6 +1818,14 @@ static struct rq *move_queued_task(struct rq *rq, struct rq_flags *rf, 
struct migration_arg {

	struct task_struct		*task;

	int				dest_cpu;

	struct set_affinity_pending	*pending;

};



struct set_affinity_pending {

	refcount_t		refs;

	struct completion	done;

	struct cpu_stop_work	stop_work;

	struct migration_arg	arg;

};



/*
@@ -1838,16 +1857,19 @@ static struct rq *__migrate_task(struct rq *rq, struct rq_flags *rf, 
 */

static int migration_cpu_stop(void *data)

{

	struct set_affinity_pending *pending;

	struct migration_arg *arg = data;

	struct task_struct *p = arg->task;

	int dest_cpu = arg->dest_cpu;

	struct rq *rq = this_rq();

	bool complete = false;

	struct rq_flags rf;



	/*

	 * The original target CPU might have gone down and we might

	 * be on another CPU but it doesn't matter.

	 */

	local_irq_disable();

	local_irq_save(rf.flags);

	/*

	 * We need to explicitly wake pending tasks before running

	 * __migrate_task() such that we will not miss enforcing cpus_ptr
@@ -1857,21 +1879,83 @@ static int migration_cpu_stop(void *data) 


	raw_spin_lock(&p->pi_lock);

	rq_lock(rq, &rf);



	pending = p->migration_pending;

	/*

	 * If task_rq(p) != rq, it cannot be migrated here, because we're

	 * holding rq->lock, if p->on_rq == 0 it cannot get enqueued because

	 * we're holding p->pi_lock.

	 */

	if (task_rq(p) == rq) {

		if (is_migration_disabled(p))

			goto out;



		if (pending) {

			p->migration_pending = NULL;

			complete = true;

		}



		/* migrate_enable() --  we must not race against SCA */

		if (dest_cpu < 0) {

			/*

			 * When this was migrate_enable() but we no longer

			 * have a @pending, a concurrent SCA 'fixed' things

			 * and we should be valid again. Nothing to do.

			 */

			if (!pending) {

				WARN_ON_ONCE(!is_cpu_allowed(p, cpu_of(rq)));

				goto out;

			}



			dest_cpu = cpumask_any_distribute(&p->cpus_mask);

		}



		if (task_on_rq_queued(p))

			rq = __migrate_task(rq, &rf, p, arg->dest_cpu);

			rq = __migrate_task(rq, &rf, p, dest_cpu);

		else

			p->wake_cpu = arg->dest_cpu;

			p->wake_cpu = dest_cpu;



	} else if (dest_cpu < 0) {

		/*

		 * This happens when we get migrated between migrate_enable()'s

		 * preempt_enable() and scheduling the stopper task. At that

		 * point we're a regular task again and not current anymore.

		 *

		 * A !PREEMPT kernel has a giant hole here, which makes it far

		 * more likely.

		 */



		/*

		 * When this was migrate_enable() but we no longer have an

		 * @pending, a concurrent SCA 'fixed' things and we should be

		 * valid again. Nothing to do.

		 */

		if (!pending) {

			WARN_ON_ONCE(!is_cpu_allowed(p, cpu_of(rq)));

			goto out;

		}

	rq_unlock(rq, &rf);

	raw_spin_unlock(&p->pi_lock);



	local_irq_enable();

		/*

		 * When migrate_enable() hits a rq mis-match we can't reliably

		 * determine is_migration_disabled() and so have to chase after

		 * it.

		 */

		task_rq_unlock(rq, p, &rf);

		stop_one_cpu_nowait(task_cpu(p), migration_cpu_stop,

				    &pending->arg, &pending->stop_work);

		return 0;

	}

out:

	task_rq_unlock(rq, p, &rf);



	if (complete)

		complete_all(&pending->done);



	/* For pending->{arg,stop_work} */

	pending = arg->pending;

	if (pending && refcount_dec_and_test(&pending->refs))

		wake_up_var(&pending->refs);



	return 0;

}
@@ -1940,6 +2024,112 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask) 
	__do_set_cpus_allowed(p, new_mask, 0);

}



/*

 * This function is wildly self concurrent, consider at least 3 times.

 */

static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flags *rf,

			    int dest_cpu, unsigned int flags)

{

	struct set_affinity_pending my_pending = { }, *pending = NULL;

	struct migration_arg arg = {

		.task = p,

		.dest_cpu = dest_cpu,

	};

	bool complete = false;



	/* Can the task run on the task's current CPU? If so, we're done */

	if (cpumask_test_cpu(task_cpu(p), &p->cpus_mask)) {

		pending = p->migration_pending;

		if (pending) {

			refcount_inc(&pending->refs);

			p->migration_pending = NULL;

			complete = true;

		}

		task_rq_unlock(rq, p, rf);



		if (complete)

			goto do_complete;



		return 0;

	}



	if (!(flags & SCA_MIGRATE_ENABLE)) {

		/* serialized by p->pi_lock */

		if (!p->migration_pending) {

			refcount_set(&my_pending.refs, 1);

			init_completion(&my_pending.done);

			p->migration_pending = &my_pending;

		} else {

			pending = p->migration_pending;

			refcount_inc(&pending->refs);

		}

	}

	pending = p->migration_pending;

	/*

	 * - !MIGRATE_ENABLE:

	 *   we'll have installed a pending if there wasn't one already.

	 *

	 * - MIGRATE_ENABLE:

	 *   we're here because the current CPU isn't matching anymore,

	 *   the only way that can happen is because of a concurrent

	 *   set_cpus_allowed_ptr() call, which should then still be

	 *   pending completion.

	 *

	 * Either way, we really should have a @pending here.

	 */

	if (WARN_ON_ONCE(!pending)) {

		task_rq_unlock(rq, p, rf);

		return -EINVAL;

	}



	if (flags & SCA_MIGRATE_ENABLE) {



		refcount_inc(&pending->refs); /* pending->{arg,stop_work} */

		task_rq_unlock(rq, p, rf);



		pending->arg = (struct migration_arg) {

			.task = p,

			.dest_cpu = -1,

			.pending = pending,

		};



		stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,

				    &pending->arg, &pending->stop_work);



		return 0;

	}



	if (task_running(rq, p) || p->state == TASK_WAKING) {



		task_rq_unlock(rq, p, rf);

		stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);



	} else {



		if (!is_migration_disabled(p)) {

			if (task_on_rq_queued(p))

				rq = move_queued_task(rq, rf, p, dest_cpu);



			p->migration_pending = NULL;

			complete = true;

		}

		task_rq_unlock(rq, p, rf);



do_complete:

		if (complete)

			complete_all(&pending->done);

	}



	wait_for_completion(&pending->done);



	if (refcount_dec_and_test(&pending->refs))

		wake_up_var(&pending->refs);



	wait_var_event(&my_pending.refs, !refcount_read(&my_pending.refs));



	return 0;

}



/*

 * Change a given task's CPU affinity. Migrate the thread to a

 * proper CPU and schedule it away if the CPU it's executing on
@@ -2009,23 +2199,8 @@ static int __set_cpus_allowed_ptr(struct task_struct *p, 
			p->nr_cpus_allowed != 1);

	}



	/* Can the task run on the task's current CPU? If so, we're done */

	if (cpumask_test_cpu(task_cpu(p), new_mask))

		goto out;

	return affine_move_task(rq, p, &rf, dest_cpu, flags);



	if (task_running(rq, p) || p->state == TASK_WAKING) {

		struct migration_arg arg = { p, dest_cpu };

		/* Need help from migration thread: drop lock and wait. */

		task_rq_unlock(rq, p, &rf);

		stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);

		return 0;

	} else if (task_on_rq_queued(p)) {

		/*

		 * OK, since we're going to drop the lock immediately

		 * afterwards anyway.

		 */

		rq = move_queued_task(rq, &rf, p, dest_cpu);

	}

out:

	task_rq_unlock(rq, p, &rf);
@@ -3205,6 +3380,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p) 
	init_numa_balancing(clone_flags, p);

#ifdef CONFIG_SMP

	p->wake_entry.u_flags = CSD_TYPE_TTWU;

	p->migration_pending = NULL;

#endif

}

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