[PATCH 2/3] sched: fix cgroup movement of forking process

[Daisuke Nishimura]

There is a small race between task_fork_fair() and sched_move_task(),
which is trying to move the parent.

        task_fork_fair()                 sched_move_task()
--------------------------------+---------------------------------
  cfs_rq = task_cfs_rq(current)
    -> cfs_rq is the "old" one.
  curr = cfs_rq->curr
    -> curr is set to the parent.
                                    task_rq_lock()
                                    dequeue_task()
                                      ->parent.se.vruntime -= (old)cfs_rq->min_vruntime
                                    enqueue_task()
                                      ->parent.se.vruntime += (new)cfs_rq->min_vruntime
                                    task_rq_unlock()
  raw_spin_lock_irqsave(rq->lock)
  se->vruntime = curr->vruntime
    -> vruntime of the child is set to that of the parent
       which has already been updated by sched_move_task().
  se->vruntime -= (old)cfs_rq->min_vruntime.
  raw_spin_unlock_irqrestore(rq->lock)

As a result, vruntime of the child becomes far bigger than expected,
if (new)cfs_rq->min_vruntime >> (old)cfs_rq->min_vruntime.

This patch fixes this problem by setting "cfs_rq" and "curr" after holding
the rq->lock.

Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
---
 kernel/sched_fair.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index df145a9..bdaa4ab 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -4787,14 +4787,17 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
  */
 static void task_fork_fair(struct task_struct *p)
 {
-	struct cfs_rq *cfs_rq = task_cfs_rq(current);
-	struct sched_entity *se = &p->se, *curr = cfs_rq->curr;
+	struct cfs_rq *cfs_rq;
+	struct sched_entity *se = &p->se, *curr;
 	int this_cpu = smp_processor_id();
 	struct rq *rq = this_rq();
 	unsigned long flags;
 
 	raw_spin_lock_irqsave(&rq->lock, flags);
 
+	cfs_rq = task_cfs_rq(current);
+	curr = cfs_rq->curr;
+
 	update_rq_clock(rq);
 
 	if (unlikely(task_cpu(p) != this_cpu)) {
-- 
1.7.1

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