sched/fair: Use task_rcu_dereference()

Simplify task_numa_compare()'s task reference magic by using
task_rcu_dereference().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Kirill Tkhai <ktkhai@parallels.com>
Cc: Kirill Tkhai <tkhai@yandex.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Link: http://lkml.kernel.org/r/20160518195733.GA15914@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -1305,6 +1305,8 @@ static void task_numa_assign(struct task_numa_env *env,
 {
 	if (env->best_task)
 		put_task_struct(env->best_task);
+	if (p)
+		get_task_struct(p);
 
 	env->best_task = p;
 	env->best_imp = imp;
@@ -1372,31 +1374,11 @@ static void task_numa_compare(struct task_numa_env *env,
 	long imp = env->p->numa_group ? groupimp : taskimp;
 	long moveimp = imp;
 	int dist = env->dist;
-	bool assigned = false;
 
 	rcu_read_lock();
-
-	raw_spin_lock_irq(&dst_rq->lock);
-	cur = dst_rq->curr;
-	/*
-	 * No need to move the exiting task or idle task.
-	 */
-	if ((cur->flags & PF_EXITING) || is_idle_task(cur))
+	cur = task_rcu_dereference(&dst_rq->curr);
+	if (cur && ((cur->flags & PF_EXITING) || is_idle_task(cur)))
 		cur = NULL;
-	else {
-		/*
-		 * The task_struct must be protected here to protect the
-		 * p->numa_faults access in the task_weight since the
-		 * numa_faults could already be freed in the following path:
-		 * finish_task_switch()
-		 *     --> put_task_struct()
-		 *         --> __put_task_struct()
-		 *             --> task_numa_free()
-		 */
-		get_task_struct(cur);
-	}
-
-	raw_spin_unlock_irq(&dst_rq->lock);
 
 	/*
 	 * Because we have preemption enabled we can get migrated around and
@@ -1479,7 +1461,6 @@ balance:
 		 */
 		if (!load_too_imbalanced(src_load, dst_load, env)) {
 			imp = moveimp - 1;
-			put_task_struct(cur);
 			cur = NULL;
 			goto assign;
 		}
@@ -1505,16 +1486,9 @@ balance:
 		env->dst_cpu = select_idle_sibling(env->p, env->dst_cpu);
 
 assign:
-	assigned = true;
 	task_numa_assign(env, cur, imp);
 unlock:
 	rcu_read_unlock();
-	/*
-	 * The dst_rq->curr isn't assigned. The protection for task_struct is
-	 * finished.
-	 */
-	if (cur && !assigned)
-		put_task_struct(cur);
 }
 
 static void task_numa_find_cpu(struct task_numa_env *env,