[PATCH] sched_rt: the task in irq context can be migrated duringcontext switching

[Chanho Min]

This issue happens under the following conditions
1. preemption is off
2. __ARCH_WANT_INTERRUPTS_ON_CTXSW is defined.
3. RT scheduling class
4. SMP system

Sequence is as follows:
1.suppose current task is A. start schedule()
2.task A is enqueued pushable task at the entry of schedule()
   __schedule
    prev = rq->curr;
    ...
    put_prev_task
     put_prev_task_rt
      enqueue_pushable_task
4.pick the task B as next task.
   next = pick_next_task(rq);
3.rq->curr set to task B and context_switch is started.
   rq->curr = next;
4.At the entry of context_swtich, release this cpu's rq->lock.
   context_switch
    prepare_task_switch
     prepare_lock_switch
      raw_spin_unlock_irq(&rq->lock);
5.Shortly after rq->lock is released, interrupt is occurred and start
IRQ context
6.try_to_wake_up() which called by ISR acquires rq->lock
    try_to_wake_up
     ttwu_remote
      rq = __task_rq_lock(p)
      ttwu_do_wakeup(rq, p, wake_flags);
        task_woken_rt
7.push_rt_task picks the task A which is enqueued before.
   task_woken_rt
    push_rt_tasks(rq)
     next_task = pick_next_pushable_task(rq)
8.At find_lock_lowest_rq(), If double_lock_balance() returns 0,
lowest_rq can be the remote rq.
  (But,If preemption is on, double_lock_balance always return 1 and it
does't happen.)
   push_rt_task
    find_lock_lowest_rq
     if (double_lock_balance(rq, lowest_rq))..
9.find_lock_lowest_rq return the available rq. task A is migrated to
the remote cpu/rq.
   push_rt_task
    ...
    deactivate_task(rq, next_task, 0);
    set_task_cpu(next_task, lowest_rq->cpu);
    activate_task(lowest_rq, next_task, 0);
10. But, task A is on irq context at this cpu.
    So, task A is scheduled by two cpus at the same time until restore from IRQ.
    Task A's stack is corrupted. Unexpected problem is occurred.

In recent ARM, I saw the patch to remove
__ARCH_WANT_INTERRUPTS_ON_CTXSW is posted.
But, if that feature is adopted by the others architecture or to
remain in the release version,,this can occur.
This is my patch to fix it. Any opinions will be appreciated.

Signed-off-by: Chanho Min <chanho.min@xxxxxxx>
---
 kernel/sched_rt.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 583a136..59e66e3 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -1388,6 +1388,11 @@ static int push_rt_task(struct rq *rq)
        if (!next_task)
                return 0;

+#ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW
+       if (unlikely(task_running(rq, next_task)))
+               return 0;
+#endif
+
 retry:
        if (unlikely(next_task == rq->curr)) {
                WARN_ON(1);
--
--
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