[bug report] drm/scheduler: rework job destruction

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

Hello Christian König,

The patch 5918045c4ed4: "drm/scheduler: rework job destruction" from
Apr 18, 2019, leads to the following static checker warning:

	drivers/gpu/drm/scheduler/sched_main.c:297 drm_sched_job_timedout()
	error: potential NULL dereference 'job'.

drivers/gpu/drm/scheduler/sched_main.c
   279  static void drm_sched_job_timedout(struct work_struct *work)
   280  {
   281          struct drm_gpu_scheduler *sched;
   282          struct drm_sched_job *job;
   283          unsigned long flags;
   284  
   285          sched = container_of(work, struct drm_gpu_scheduler, work_tdr.work);
   286          job = list_first_entry_or_null(&sched->ring_mirror_list,
   287                                         struct drm_sched_job, node);
   288  
   289          if (job)
                    ^^^
We assume that job can be NULL.

   290                  job->sched->ops->timedout_job(job);
   291  
   292          /*
   293           * Guilty job did complete and hence needs to be manually removed
   294           * See drm_sched_stop doc.
   295           */
   296          if (sched->free_guilty) {

Originally (last week) this check was "if (list_empty(&job->node))"
which is obviously problematic if job is NULL.  It's not clear to me
that this new check ensures that job is non-NULL either.

   297                  job->sched->ops->free_job(job);
                        ^^^^^
Dereference.

   298                  sched->free_guilty = false;
   299          }
   300  
   301          spin_lock_irqsave(&sched->job_list_lock, flags);
   302          drm_sched_start_timeout(sched);
   303          spin_unlock_irqrestore(&sched->job_list_lock, flags);
   304  }

regards,
dan carpenter
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