sched: Simplify migration_cpu_stop()

Since, when ->stop_pending, only the stopper can uninstall
p->migration_pending. This could simplify a few ifs, because:

  (pending != NULL) => (pending == p->migration_pending)

Also, the fatty comment above affine_move_task() probably needs a bit
of gardening.
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -1926,6 +1926,12 @@ static int migration_cpu_stop(void *data)
 	raw_spin_lock(&p->pi_lock);
 	rq_lock(rq, &rf);
 
+	/*
+	 * If we were passed a pending, then ->stop_pending was set, thus
+	 * p->migration_pending must have remained stable.
+	 */
+	WARN_ON_ONCE(pending && 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
@@ -1936,7 +1942,6 @@ static int migration_cpu_stop(void *data)
 			goto out;
 
 		if (pending) {
-			if (p->migration_pending == pending)
 			p->migration_pending = NULL;
 			complete = true;
 		}
@@ -1976,7 +1981,6 @@ static int migration_cpu_stop(void *data)
 		 * somewhere allowed, we're done.
 		 */
 		if (cpumask_test_cpu(task_cpu(p), p->cpus_ptr)) {
-			if (p->migration_pending == pending)
 			p->migration_pending = NULL;
 			complete = true;
 			goto out;
@@ -2165,16 +2169,21 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
  *
  * (1) In the cases covered above. There is one more where the completion is
  * signaled within affine_move_task() itself: when a subsequent affinity request
- * cancels the need for an active migration. Consider:
+ * occurs after the stopper bailed out due to the targeted task still being
+ * Migrate-Disable. Consider:
  *
  *     Initial conditions: P0->cpus_mask = [0, 1]
  *
- *     P0@CPU0            P1                             P2
- *
+ *     CPU0		  P1				P2
+ *     <P0>
  *       migrate_disable();
  *       <preempted>
  *                        set_cpus_allowed_ptr(P0, [1]);
  *                          <blocks>
+ *     <migration/0>
+ *       migration_cpu_stop()
+ *         is_migration_disabled()
+ *           <bails>
  *                                                       set_cpus_allowed_ptr(P0, [0, 1]);
  *                                                         <signal completion>
  *                          <awakes>