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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

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?

Regards,

Tvrtko