Commit c20cf065 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

sched: Simplify migration_cpu_stop() 



When affine_move_task() issues a migration_cpu_stop(), the purpose of
that function is to complete that @pending, not any random other
p->migration_pending that might have gotten installed since.

This realization much simplifies migration_cpu_stop() and allows
further necessary steps to fix all this as it provides the guarantee
that @pending's stopper will complete @pending (and not some random
other @pending).

Fixes: 6d337eab ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
Cc: stable@kernel.org
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Reviewed-by: default avatarValentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210224131355.430014682@infradead.org
parent 8a6edb52

kernel/sched/core.c

+8 −48
Original line number Diff line number Diff line
@@ -1898,8 +1898,8 @@ 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 set_affinity_pending *pending = arg->pending;

	struct task_struct *p = arg->task;

	int dest_cpu = arg->dest_cpu;

	struct rq *rq = this_rq();
@@ -1921,25 +1921,6 @@ static int migration_cpu_stop(void *data) 
	raw_spin_lock(&p->pi_lock);

	rq_lock(rq, &rf);



	pending = p->migration_pending;

	if (pending && !arg->pending) {

		/*

		 * This happens from sched_exec() and migrate_task_to(),

		 * neither of them care about pending and just want a task to

		 * maybe move about.

		 *

		 * Even if there is a pending, we can ignore it, since

		 * affine_move_task() will have it's own stop_work's in flight

		 * which will manage the completion.

		 *

		 * Notably, pending doesn't need to match arg->pending. This can

		 * happen when tripple concurrent affine_move_task() first sets

		 * pending, then clears pending and eventually sets another

		 * pending.

		 */

		pending = NULL;

	}



	/*

	 * 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
@@ -1950,31 +1931,20 @@ static int migration_cpu_stop(void *data) 
			goto out;



		if (pending) {

			if (p->migration_pending == 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(!cpumask_test_cpu(task_cpu(p), &p->cpus_mask));

				goto out;

			}



		if (dest_cpu < 0)

			dest_cpu = cpumask_any_distribute(&p->cpus_mask);

		}



		if (task_on_rq_queued(p))

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

		else

			p->wake_cpu = dest_cpu;



	} else if (dest_cpu < 0 || pending) {

	} else if (pending) {

		/*

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

		 * preempt_enable() and scheduling the stopper task. At that
@@ -1989,22 +1959,13 @@ static int migration_cpu_stop(void *data) 
		 * ->pi_lock, so the allowed mask is stable - if it got

		 * somewhere allowed, we're done.

		 */

		if (pending && cpumask_test_cpu(task_cpu(p), p->cpus_ptr)) {

		if (cpumask_test_cpu(task_cpu(p), p->cpus_ptr)) {

			if (p->migration_pending == pending)

				p->migration_pending = NULL;

			complete = true;

			goto out;

		}



		/*

		 * 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(!cpumask_test_cpu(task_cpu(p), &p->cpus_mask));

			goto out;

		}



		/*

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

		 * determine is_migration_disabled() and so have to chase after
@@ -2022,7 +1983,6 @@ static int migration_cpu_stop(void *data) 
		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);