In the Linux kernel, the following vulnerability has been resolved:

sched/core: Call wakeup_preempt() for queued tasks during class change

Paul reported recent RCU changes exposed a lost resched where CPU idles
with a rcutorture thread waiting while being completely runnable.

Dumping the rq state at the time of idling with runnable tasks showed
the following:

  rq->next_class = &fair_sched_class
  rq->nr_running = 1
  rq->cfs.h_nr_queued = 0
  rq->rt.rt_nr_running = 1

Although the waiting task was an RT task, "rq->next_class" did not point
to rt_sched_class and was stuck at the lower priority fair_sched_class.

Inspecting sched_change_end() revealed the following race with
__sched_setscheduler() for queued, but non-running tasks:

    CPU0                                   CPU1
    ====                                   ====

  __schedule()

    /*
     * rq->donor is a fair task that blocks.
     * rq->next_class = &fair_sched_class
     */

    rq_lock(rq0)

    pick_next_task()
      /* nr_running = 0 */
      pick_task_fair()
        sched_balance_newidle()
          raw_spin_rq_unlock(rq0)
                                    rq_lock(rq0)

                                    activate_task(rq0, p)
                                      /* p is a fair task */
                                      rq0->nr_running++;
                                      rq0->cfs.h_nr_queued++;
                                      wakeup_preempt(rq, p)
                                        rq->next_class = &fair_sched_class
                                        resched_curr()

                                    rq_unlock(rq0)

                                    __sched_setscheduler(p)
                                      /* p is moved to SCHED_RR */
                                      task_rq_lock(p)
                                      sched_change_begin()

				      rq0->cfs.h_nr_queued--;
				      rq0->rt.rt_nr_running++;

                                      sched_change_end()
                                        /*
                                         * p->on_rq = 1 (ctx->queued)
                                         * but p is not rq->donor
                                         * (!ctx->running) hence
                                         * resched_curr() is and
                                         * rq->next_class upgrade is
                                         * skipped.
                                         */

                                       task_rq_unlock()

          raw_spin_rq_lock(rq0)

          /*
           * rq->nr_running = 1
           * rq->cfs.h_nr_queued = 0
           * rq->rt.rt_nr_running = 1
           * rq->next_class = &fair_sched_class
           */

           if (rq->cfs.h_nr_queued && ...)
             /* Above is false */

           if (rq_modified_above(this_rq, &fair_sched_class))
             /* Also false since rq->next_class is still fair */

      /*
       * sched_balance_newidle() is considered to
       * have failed to pull any fair tasks and no
       * other wakeups were detected.
       */
       return NULL; /* pick_task_fair() */

    pick_task_idle()
      return rq->idle;

    next = rq->idle;

    /* Preemption indicators are lost */
    clear_tsk_need_resched(prev);
    clear_preempt_need_resched();

    rq->curr = rq->idle;

  !!! rq->idle switches in with rq->running == 1 but !need_resched() !!!

In simple words, enqueue, followed by a class change of the newly queued
task goes unnoticed in rq->next_class indicator and
sched_balance_newidle() sees no fair task, nor any higher priority task
enqueued.

The above sequence will lead to CPU idling with a runnable task on the
CPU.

Call wakeup_preempt() for ctx->queued cases in sched_change_end() to
re-evaluate rq->next_class after class change (ENQUEUE_CLASS) and set
the NEED_RESCHED indicator accordingly.

Newidle balance will correctly see an enqueue of a higher priority
t
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