Re: [PATCH v2] drm/scheduler: Fix hang when sched_entity released

[Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>]

On 2/18/21 10:15 AM, Christian König wrote:
> Am 18.02.21 um 16:05 schrieb Andrey Grodzovsky:
>>
>> On 2/18/21 3:07 AM, Christian König wrote:
>>>
>>>
>>> Am 17.02.21 um 22:59 schrieb Andrey Grodzovsky:
>>>> Problem: If scheduler is already stopped by the time sched_entity
>>>> is released and entity's job_queue not empty I encountred
>>>> a hang in drm_sched_entity_flush. This is because drm_sched_entity_is_idle
>>>> never becomes false.
>>>>
>>>> Fix: In drm_sched_fini detach all sched_entities from the
>>>> scheduler's run queues. This will satisfy drm_sched_entity_is_idle.
>>>> Also wakeup all those processes stuck in sched_entity flushing
>>>> as the scheduler main thread which wakes them up is stopped by now.
>>>>
>>>> v2:
>>>> Reverse order of drm_sched_rq_remove_entity and marking
>>>> s_entity as stopped to prevent reinserion back to rq due
>>>> to race.
>>>>
>>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
>>>> ---
>>>>   drivers/gpu/drm/scheduler/sched_main.c | 31 +++++++++++++++++++++++++++++++
>>>>   1 file changed, 31 insertions(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c 
>>>> b/drivers/gpu/drm/scheduler/sched_main.c
>>>> index 908b0b5..c6b7947 100644
>>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>>> @@ -897,9 +897,40 @@ EXPORT_SYMBOL(drm_sched_init);
>>>>    */
>>>>   void drm_sched_fini(struct drm_gpu_scheduler *sched)
>>>>   {
>>>> +    int i;
>>>> +    struct drm_sched_entity *s_entity;
>>>
>>> BTW: Please order that so that i is declared last.
>>>
>>>>       if (sched->thread)
>>>>           kthread_stop(sched->thread);
>>>>   +    /* Detach all sched_entites from this scheduler once it's stopped */
>>>> +    for (i = DRM_SCHED_PRIORITY_COUNT - 1; i >= DRM_SCHED_PRIORITY_MIN; 
>>>> i--) {
>>>> +        struct drm_sched_rq *rq = &sched->sched_rq[i];
>>>> +
>>>> +        if (!rq)
>>>> +            continue;
>>>> +
>>>> +        /* Loop this way because rq->lock is taken in 
>>>> drm_sched_rq_remove_entity */
>>>> +        spin_lock(&rq->lock);
>>>> +        while ((s_entity = list_first_entry_or_null(&rq->entities,
>>>> +                                struct drm_sched_entity,
>>>> +                                list))) {
>>>> +            spin_unlock(&rq->lock);
>>>> +
>>>> +            /* Prevent reinsertion and remove */
>>>> +            spin_lock(&s_entity->rq_lock);
>>>> +            s_entity->stopped = true;
>>>> +            drm_sched_rq_remove_entity(rq, s_entity);
>>>> +            spin_unlock(&s_entity->rq_lock);
>>>
>>> Well this spin_unlock/lock dance here doesn't look correct at all now.
>>>
>>> Christian.
>>
>>
>> In what way ? It's in the same same order as in other call sites (see 
>> drm_sched_entity_push_job and drm_sched_entity_flush).
>> If i just locked rq->lock and did list_for_each_entry_safe while manually 
>> deleting entity->list instead of calling
>> drm_sched_rq_remove_entity this still would not be possible as the order of 
>> lock acquisition between  s_entity->rq_lock
>> and rq->lock would be reverse compared to the call sites mentioned above.
>
> Ah, now I understand. You need this because drm_sched_rq_remove_entity() will 
> grab the rq lock again!
>
> Problem is now what prevents the entity from being destroyed while you remove it?
>
> Christian.

Right, well, since (unfortunately) sched_entity is part of amdgpu_ctx_entity and 
amdgpu_ctx_entity is refcounted
there is a problem here that we don't increment amdgpu_ctx.refcount when 
assigning  sched_entity
to new rq (e.g. before drm_sched_rq_add_entity) and not decrement before 
removing. We do it for
amdgpu_cs_parser.entity for example (in amdgpu_cs_parser_init and 
amdgpu_cs_parser_fini by
calling amdgpu_ctx_get and amdgpu_ctx_put). But this seems a bit tricky due to 
all the drm_sched_entity_select_rq
logic.

Another, kind of a band aid fix, would probably be just locking 
amdgpu_ctx_mgr.lock around drm_sched_fini
when finalizing the fence driver and around idr iteration in amdgpu_ctx_mgr_fini 
(which should be lock protected
anyway as I see from other idr usages in the code) ... This should prevent this 
use after free.

Andrey


>
>>
>> Andrey
>>
>>
>>
>>>
>>>> +
>>>> +            spin_lock(&rq->lock);
>>>> +        }
>>>> +        spin_unlock(&rq->lock);
>>>> +
>>>> +    }
>>>> +
>>>> +    /* Wakeup everyone stuck in drm_sched_entity_flush for this scheduler */
>>>> +    wake_up_all(&sched->job_scheduled);
>>>> +
>>>>       /* Confirm no work left behind accessing device structures */
>>>>       cancel_delayed_work_sync(&sched->work_tdr);
>>>
>
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