sched: Collate affine_move_task() stoppers

The SCA_MIGRATE_ENABLE and task_running() cases are almost identical,
collapse them to avoid further duplication.

Fixes: 6d337ea ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
Cc: stable@kernel.org
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210224131355.500108964@infradead.org

Author: Peter Zijlstra

kernel/sched/core.c

@@ -2239,30 +2239,23 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
 		return -EINVAL;
 	}
 
-	if (flags & SCA_MIGRATE_ENABLE) {
-
-		refcount_inc(&pending->refs); /* pending->{arg,stop_work} */
-		p->migration_flags &= ~MDF_PUSH;
-		task_rq_unlock(rq, p, rf);
-
-		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) {
 		/*
-		 * Lessen races (and headaches) by delegating
-		 * is_migration_disabled(p) checks to the stopper, which will
-		 * run on the same CPU as said p.
+		 * MIGRATE_ENABLE gets here because 'p == current', but for
+		 * anything else we cannot do is_migration_disabled(), punt
+		 * and have the stopper function handle it all race-free.
 		 */
+
 		refcount_inc(&pending->refs); /* pending->{arg,stop_work} */
+		if (flags & SCA_MIGRATE_ENABLE)
+			p->migration_flags &= ~MDF_PUSH;
 		task_rq_unlock(rq, p, rf);
 
 		stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,
 				    &pending->arg, &pending->stop_work);
 
+		if (flags & SCA_MIGRATE_ENABLE)
+			return 0;
 	} else {
 
 		if (!is_migration_disabled(p)) {