Re: [PATCH v4] drm/scheduler: Avoid accessing freed bad job.

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 06.02.20 um 12:10 schrieb Lucas Stach:
> Hi all,
>
> On Mi, 2020-02-05 at 19:24 +0100, Lucas Stach wrote:
>> Hi Andrey,
>>
>> This commit breaks all drivers, which may bail out of the timeout
>> processing as they wish to extend the timeout (etnaviv, v3d).
>>
>> Those drivers currently just return from the timeout handler before
>> calling drm_sched_stop(), which means with this commit applied we are
>> removing the first job from the ring_mirror_list, but never put it
>> back. This leads to jobs getting lost from the ring mirror, which then
>> causes quite a bit of fallout like unsignaled fences.
>>
>> Not sure yet what to do about it, we can either add a function to add
>> the job back to the ring_mirror if the driver wants to extend the
>> timeout, or we could look for another way to stop
>> drm_sched_cleanup_jobs from freeing jobs that are currently in timeout
>> processing.
> So after thinking about this a bit more my opinion is that we need to
> revert this change for now and go back to the drawing board for the
> scheduler timeout handling.
>
> Right now this starts to feel like a big midlayer mistake with all the
> very intricate intertwining between the drivers and the scheduler. The
> rules on when it's safe to manipulate the ring mirror and when
> completed jobs are signaled and freed are not really well specified.
> The fact that we need to mutate state in order to get rid of races
> instead of having a single big "timeout processing is owner of the
> scheduler state for now" is a big fat warning sign IMHO.

Yes, that strongly feels like a hack to me as well. But I didn't had 
time and still haven't to take a closer look and suggest something better.

Christian.

>
> It took me far longer than I'd like to admit to understand the failure
> mode with fences not getting signaled after a GPU hang. The back and
> forth between scheduler and driver code makes things really hard to
> follow.
>
> Regards,
> Lucas
>
>> Regards,
>> Lucas
>>
>> On Mo, 2019-11-25 at 15:51 -0500, Andrey Grodzovsky wrote:
>>> Problem:
>>> Due to a race between drm_sched_cleanup_jobs in sched thread and
>>> drm_sched_job_timedout in timeout work there is a possiblity that
>>> bad job was already freed while still being accessed from the
>>> timeout thread.
>>>
>>> Fix:
>>> Instead of just peeking at the bad job in the mirror list
>>> remove it from the list under lock and then put it back later when
>>> we are garanteed no race with main sched thread is possible which
>>> is after the thread is parked.
>>>
>>> v2: Lock around processing ring_mirror_list in drm_sched_cleanup_jobs.
>>>
>>> v3: Rebase on top of drm-misc-next. v2 is not needed anymore as
>>> drm_sched_get_cleanup_job already has a lock there.
>>>
>>> v4: Fix comments to relfect latest code in drm-misc.
>>>
>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
>>> Reviewed-by: Christian König <christian.koenig@amd.com>
>>> Tested-by: Emily Deng <Emily.Deng@amd.com>
>>> ---
>>>   drivers/gpu/drm/scheduler/sched_main.c | 27 +++++++++++++++++++++++++++
>>>   1 file changed, 27 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>> index 6774955..1bf9c40 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>> @@ -284,10 +284,21 @@ static void drm_sched_job_timedout(struct work_struct *work)
>>>   	unsigned long flags;
>>>   
>>>   	sched = container_of(work, struct drm_gpu_scheduler, work_tdr.work);
>>> +
>>> +	/* Protects against concurrent deletion in drm_sched_get_cleanup_job */
>>> +	spin_lock_irqsave(&sched->job_list_lock, flags);
>>>   	job = list_first_entry_or_null(&sched->ring_mirror_list,
>>>   				       struct drm_sched_job, node);
>>>   
>>>   	if (job) {
>>> +		/*
>>> +		 * Remove the bad job so it cannot be freed by concurrent
>>> +		 * drm_sched_cleanup_jobs. It will be reinserted back after sched->thread
>>> +		 * is parked at which point it's safe.
>>> +		 */
>>> +		list_del_init(&job->node);
>>> +		spin_unlock_irqrestore(&sched->job_list_lock, flags);
>>> +
>>>   		job->sched->ops->timedout_job(job);
>>>   
>>>   		/*
>>> @@ -298,6 +309,8 @@ static void drm_sched_job_timedout(struct work_struct *work)
>>>   			job->sched->ops->free_job(job);
>>>   			sched->free_guilty = false;
>>>   		}
>>> +	} else {
>>> +		spin_unlock_irqrestore(&sched->job_list_lock, flags);
>>>   	}
>>>   
>>>   	spin_lock_irqsave(&sched->job_list_lock, flags);
>>> @@ -370,6 +383,20 @@ void drm_sched_stop(struct drm_gpu_scheduler *sched, struct drm_sched_job *bad)
>>>   	kthread_park(sched->thread);
>>>   
>>>   	/*
>>> +	 * Reinsert back the bad job here - now it's safe as
>>> +	 * drm_sched_get_cleanup_job cannot race against us and release the
>>> +	 * bad job at this point - we parked (waited for) any in progress
>>> +	 * (earlier) cleanups and drm_sched_get_cleanup_job will not be called
>>> +	 * now until the scheduler thread is unparked.
>>> +	 */
>>> +	if (bad && bad->sched == sched)
>>> +		/*
>>> +		 * Add at the head of the queue to reflect it was the earliest
>>> +		 * job extracted.
>>> +		 */
>>> +		list_add(&bad->node, &sched->ring_mirror_list);
>>> +
>>> +	/*
>>>   	 * Iterate the job list from later to  earlier one and either deactive
>>>   	 * their HW callbacks or remove them from mirror list if they already
>>>   	 * signaled.
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