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

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Hi,

On 13.07.2023 09:39, Tvrtko Ursulin wrote:

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).

Engine is not ref counted (at least directly), hardware engines seems to be even persistent across whole life of i915. I guess before introduction of virtual engines the assumption about validity of rq->engine was correct and developers used it to access rq->engine->gt, rq->engine->i915, etc
So the problems described here are probably leftovers of this change.
After virtual engines were introduced "is_power_of_2(rq->execution_mask)" was used to detect rq->engine is virtual. And after adding parallel engines it does not seem to be valid check anymore.

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.

I guess:
- setting engine to NULL in remove_from_context,
- caching gt pointer,
- checking for null pointer in i915_fence_release

should be enough to solve current issue. However I am not sure if there are no more dragons hidden in other execution paths. I will try inspect the code. Btw there is rq->i915 but code only uses "rq->engine->i915" which way shall we go? remove former or latter?

Regards
Andrzej


Regards,

Tvrtko




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