[bug report] sched: Fix priority inversion of cookied task with sibling

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Joel Fernandes (Google),

This is a semi-automatic email about new static checker warnings.

The patch 7afbba119f0d: "sched: Fix priority inversion of cookied 
task with sibling" from Nov 17, 2020, leads to the following Smatch 
complaint:

    kernel/sched/core.c:5492 pick_next_task()
    error: we previously assumed 'next' could be null (see line 5488)

kernel/sched/core.c
  5481          /*
  5482           * Optimize for common case where this CPU has no cookies
  5483           * and there are no cookied tasks running on siblings.
  5484           */
  5485          if (!need_sync) {
  5486                  for_each_class(class) {
  5487                          next = class->pick_task(rq);
  5488                          if (next)
  5489                                  break;

Is it possible to go through the whole loop withint finding a next?

  5490                  }
  5491  
  5492                  if (!next->core_cookie) {
                             ^^^^^^^^^^^^^^^^^
I assume not, but I don't read the code, I just forward the warnings
on.  I am a bot.  Beep Boop!

  5493                          rq->core_pick = NULL;
  5494                          /*
  5495                           * For robustness, update the min_vruntime_fi for
  5496                           * unconstrained picks as well.
  5497                           */
  5498                          WARN_ON_ONCE(fi_before);
  5499                          task_vruntime_update(rq, next, false);
  5500                          goto done;
  5501                  }
  5502          }
  5503  
  5504          for_each_cpu(i, smt_mask) {
  5505                  struct rq *rq_i = cpu_rq(i);
  5506  

regards,
dan carpenter