[Intel-gfx] [PATCH v2] drm/i915/gt: update request engine before removing virtual GuC engine

[Tvrtko Ursulin]

On 12/07/2023 19:54, John Harrison wrote:
> On 7/12/2023 09:27, Andrzej Hajda wrote:
>> On 12.07.2023 14:35, Tvrtko Ursulin wrote:
>>> On 12/07/2023 13:18, Andrzej Hajda wrote:
>>>> On 11.07.2023 17:27, Tvrtko Ursulin wrote:
>>>>> On 11/07/2023 14:58, Andrzej Hajda wrote:
>>>>>> On 11.07.2023 13:34, Andi Shyti wrote:
>>>>>>> Hi Andrzej,
>>>>>>>
>>>>>>>> drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c | 11 +++++++++++
>>>>>>>>           1 file changed, 11 insertions(+)
>>>>>>>>
>>>>>>>>          diff --git 
>>>>>>>> a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c 
>>>>>>>> b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
>>>>>>>>          index a0e3ef1c65d246..2c877ea5eda6f0 100644
>>>>>>>>          --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
>>>>>>>>          +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
>>>>>>>>          @@ -3461,6 +3461,8 @@ static void guc_prio_fini(struct 
>>>>>>>> i915_request *rq, struct intel_context *ce)
>>>>>>>>           static void remove_from_context(struct i915_request *rq)
>>>>>>>>           {
>>>>>>>>                  struct intel_context *ce = 
>>>>>>>> request_to_scheduling_context(rq);
>>>>>>>>          +       struct intel_engine_cs *engine;
>>>>>>>>          +       intel_engine_mask_t tmp;
>>>>>>>>
>>>>>>>> GEM_BUG_ON(intel_context_is_child(ce));
>>>>>>>>
>>>>>>>>          @@ -3478,6 +3480,15 @@ static void 
>>>>>>>> remove_from_context(struct i915_request *rq)
>>>>>>>>
>>>>>>>>                  atomic_dec(&ce->guc_id.ref);
>>>>>>>> i915_request_notify_execute_cb_imm(rq);
>>>>>>>>          +
>>>>>>>>          +       /*
>>>>>>>>          +        * GuC virtual engine can disappear after this 
>>>>>>>> call, so let's assign
>>>>>>>>          +        * something valid, as driver expects this to 
>>>>>>>> be always valid pointer.
>>>>>>>>          +        */
>>>>>>>>          +       for_each_engine_masked(engine, rq->engine->gt, 
>>>>>>>> rq->execution_mask, tmp) {
>>>>>>>>          +               rq->engine = engine;
>>>>>>>>
>>>>>>>>      yes... here the context might lose the virtual engine... I 
>>>>>>>> wonder
>>>>>>>>      whether this is the rigth solution, though. Maybe we should 
>>>>>>>> set
>>>>>>>>      rq->engine = NULL; and check for NULL? Don't know.
>>>>>>>>
>>>>>>>>
>>>>>>>> Setting NULL causes occasional null page de-reference in
>>>>>>>>
>>>>>>>> i915_request_wait_timeout:
>>>>>>>>
>>>>>>>> mutex_release(&rq->engine->gt->reset.mutex.dep_map, _THIS_IP_)
>>>>>>>>
>>>>>>>> rq->engine after removing rq from context is (IMHO) used as a 
>>>>>>>> set of aliases
>>>>>>>> for gt and i915 (despite rq itself contains the alias to i915).
>>>>>>> without investigating further, but maybe that code is not even
>>>>>>> supposed to be executed, at this point, if the request's assigned
>>>>>>> virtual engine is removed.
>>>>>>
>>>>>> Real tests show it is executed and the function 
>>>>>> i915_request_wait_timeout is quite generic
>>>>>> I guess it is quite typical use-case, the only question is about 
>>>>>> timings - what happens earlier -
>>>>>> finalization of i915_request_wait_timeout or context removal.
>>>>>>
>>>>>> The other point rq->engine is accessed after context removal is 
>>>>>> i915_fence_release -
>>>>>> there is long comment there regarding virtual context and reuse 
>>>>>> retired rq.
>>>>>> Anyway calling there "intel_engine_is_virtual(rq->engine)" is 
>>>>>> risky without this patch and KASAN complains clearly about it:
>>>>>> http://gfx-ci.igk.intel.com/tree/drm-tip/kasan.html?testfilter=gem_exec_balancer
>>>>>
>>>>> Looks like a bug introduced in bcb9aa45d5a0 ("Revert "drm/i915: 
>>>>> Hold reference to intel_context over life of i915_request""), which 
>>>>> was a partial revert of 1e98d8c52ed5 ("drm/i915: Hold reference to 
>>>>> intel_context over life of i915_request").
>>>>>
>>>>> Ie. if 1e98d8c52ed5 recognised the problem with disappearing 
>>>>> rq->engine, then I am confused how bcb9aa45d5a0 left the rq->engine 
>>>>> dereference in there after removing the extra reference.
>>>>>
>>>>> Could it be that the intel_engine_is_virtual check simply needs to 
>>>>> be removed from i915_fence_release, restoring things to how they 
>>>>> were before 1e98d8c52ed5? Could you try it out?
>>>>
>>>>
>>>> I have already tried something similar [1] and KASAN bugs 
>>>> disappeared, or more precisely gem_exec_balance tests passed. But I 
>>>> have been warned by Nirmoy guc virtual engines can be created for 
>>>> only one real engine (ie. is_power_of_2(rq->execution_mask) is true 
>>>> but rq->engine points to virtual engine).
>>>>
>>>> [1]: https://patchwork.freedesktop.org/series/118879/
>>>
>>> Ugh.. Try involving media umd folks to see if they need a single 
>>> engine virtual engine? Or we could always just not create it in the 
>>> driver, I mean just use the physical one.
>>
>>
>> In case there is single physical engine intel_engine_create_virtual 
>> falls back to intel_context_create (no virtual engine), but in case of 
>> parallel contexts there is special KMD flag FORCE_VIRTUAL which 
>> enforces virtual engine even for single physical engine. So it seems 
>> to be KMD concept.
>>
>> Anyway is it worth investigating how to make 
>> "is_power_of_2(rq->execution_mask)" indication of dangling engine 
>> pointer? It will not help in 1st case:
>> mutex_release(&rq->engine->gt->reset.mutex.dep_map, _THIS_IP_)
>>
>>
> There seems to be a fundamental problem here. Object 1 (rq) is holding a 
> pointer to a reference counted and transient object 2 (engine) but 
> without taking a reference count for itself. That is a Bad Thing(tm). 
> I'm not following the description in the revert patch as to why rq can't 
> ref count the context/engine. Is there actually a recursive counting 
> problem? Or is it just a lifetime issue caused by requests hanging 
> around indefinitely because they are locked by a user process?
> 
> Either way, jumping through convoluted hoops to ensure the code does not 
> attempt to dereference a dangling pointer seems like the wrong fix. 
> Removing the engine pointer when the request is completed and no longer 
> dependent upon an engine (but before the engine can possibly be 
> destroyed) seems like a much better solution. And then making the 
> request handling code check for and cope with a null engine pointer. It 
> sounds like the only problem there is the above mutex, but there is an 
> alternate route to that? Although why a completed request would need 
> access to a GT reset mutex seems confusing. If the request is done, then 
> what connection does it still have to the GT?

Agreed in principle but the question is how invasive would it be to 
change the rules.

With the latest info that the issue is really just the GuC _parallel_ 
engine setup, and looking at the code, I wonder if we couldn't just flag 
the rq->flags with "kernel context request". The code in 
i915_fence_release claims the rq pool is only relevant for those so it 
sounds it would be safe to skip everything else based on that new flag.

For the mutex_release path, presumable the bad deref is only _after_ the 
wait, right? (Only once the request has been retired.)

In which case caching the gt pointer at the start of 
i915_request_wait_timeout would be sufficient.

That should be a few lines fixup overall and then the idea of allowing 
rq->engine to be reset to NULL can be explored more leisurely.

Regards,

Tvrtko