Re: [Intel-gfx] [PATCH 28/47] drm/i915: Hold reference to intel_context over life of i915_request

[John Harrison <john.c.harrison@intel.com>]

On 7/12/2021 14:36, Matthew Brost wrote:
> On Mon, Jul 12, 2021 at 08:05:30PM +0000, Matthew Brost wrote:
>> On Mon, Jul 12, 2021 at 11:23:14AM -0700, John Harrison wrote:
>>> On 6/24/2021 00:04, Matthew Brost wrote:
>>>> Hold a reference to the intel_context over life of an i915_request.
>>>> Without this an i915_request can exist after the context has been
>>>> destroyed (e.g. request retired, context closed, but user space holds a
>>>> reference to the request from an out fence). In the case of GuC
>>>> submission + virtual engine, the engine that the request references is
>>>> also destroyed which can trigger bad pointer dref in fence ops (e.g.
>>> Maybe quickly explain a why this is different for GuC submission vs
>>> execlist? Presumably it is about only decomposing virtual engines to
>>> physical ones in execlist mode?
>>>
>> Yes, it because in execlists we always end up pointing to a physical
>> engine in the end while in GuC mode we can be pointing to dynamically
>> allocated virtual engine. I can update the comment.
>>
>>>> i915_fence_get_driver_name). We could likely change
>>>> i915_fence_get_driver_name to avoid touching the engine but let's just
>>>> be safe and hold the intel_context reference.
>>>>
>>>> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
>>>> ---
>>>>    drivers/gpu/drm/i915/i915_request.c | 54 ++++++++++++-----------------
>>>>    1 file changed, 22 insertions(+), 32 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
>>>> index de9deb95b8b1..dec5a35c9aa2 100644
>>>> --- a/drivers/gpu/drm/i915/i915_request.c
>>>> +++ b/drivers/gpu/drm/i915/i915_request.c
>>>> @@ -126,39 +126,17 @@ static void i915_fence_release(struct dma_fence *fence)
>>>>    	i915_sw_fence_fini(&rq->semaphore);
>>>>    	/*
>>>> -	 * Keep one request on each engine for reserved use under mempressure
>>>> -	 *
>>>> -	 * We do not hold a reference to the engine here and so have to be
>>>> -	 * very careful in what rq->engine we poke. The virtual engine is
>>>> -	 * referenced via the rq->context and we released that ref during
>>>> -	 * i915_request_retire(), ergo we must not dereference a virtual
>>>> -	 * engine here. Not that we would want to, as the only consumer of
>>>> -	 * the reserved engine->request_pool is the power management parking,
>>>> -	 * which must-not-fail, and that is only run on the physical engines.
>>>> -	 *
>>>> -	 * Since the request must have been executed to be have completed,
>>>> -	 * we know that it will have been processed by the HW and will
>>>> -	 * not be unsubmitted again, so rq->engine and rq->execution_mask
>>>> -	 * at this point is stable. rq->execution_mask will be a single
>>>> -	 * bit if the last and _only_ engine it could execution on was a
>>>> -	 * physical engine, if it's multiple bits then it started on and
>>>> -	 * could still be on a virtual engine. Thus if the mask is not a
>>>> -	 * power-of-two we assume that rq->engine may still be a virtual
>>>> -	 * engine and so a dangling invalid pointer that we cannot dereference
>>>> -	 *
>>>> -	 * For example, consider the flow of a bonded request through a virtual
>>>> -	 * engine. The request is created with a wide engine mask (all engines
>>>> -	 * that we might execute on). On processing the bond, the request mask
>>>> -	 * is reduced to one or more engines. If the request is subsequently
>>>> -	 * bound to a single engine, it will then be constrained to only
>>>> -	 * execute on that engine and never returned to the virtual engine
>>>> -	 * after timeslicing away, see __unwind_incomplete_requests(). Thus we
>>>> -	 * know that if the rq->execution_mask is a single bit, rq->engine
>>>> -	 * can be a physical engine with the exact corresponding mask.
>>>> +	 * Keep one request on each engine for reserved use under mempressure,
>>>> +	 * do not use with virtual engines as this really is only needed for
>>>> +	 * kernel contexts.
>>>>    	 */
>>>> -	if (is_power_of_2(rq->execution_mask) &&
>>>> -	    !cmpxchg(&rq->engine->request_pool, NULL, rq))
>>>> +	if (!intel_engine_is_virtual(rq->engine) &&
>>>> +	    !cmpxchg(&rq->engine->request_pool, NULL, rq)) {
>>>> +		intel_context_put(rq->context);
>>>>    		return;
>>>> +	}
>>>> +
>>>> +	intel_context_put(rq->context);
>>> The put is actually unconditional? So it could be moved before the if?
>>>
>> Yep, I think so.
>>
> Wait nope. We reference rq->engine which could a virtual engine and the
> intel_context_put could free that engine. So we need to do the put after
> we reference it.
>
> Matt
Doh! That's a pretty good reason.

Okay, with a tweaked description to explain about virtual engines being 
different on GuC vs execlist...

Reviewed-by: John Harrison <John.C.Harrison@Intel.com>

>
>> Matt
>>
>>> John.
>>>
>>>>    	kmem_cache_free(global.slab_requests, rq);
>>>>    }
>>>> @@ -977,7 +955,18 @@ __i915_request_create(struct intel_context *ce, gfp_t gfp)
>>>>    		}
>>>>    	}
>>>> -	rq->context = ce;
>>>> +	/*
>>>> +	 * Hold a reference to the intel_context over life of an i915_request.
>>>> +	 * Without this an i915_request can exist after the context has been
>>>> +	 * destroyed (e.g. request retired, context closed, but user space holds
>>>> +	 * a reference to the request from an out fence). In the case of GuC
>>>> +	 * submission + virtual engine, the engine that the request references
>>>> +	 * is also destroyed which can trigger bad pointer dref in fence ops
>>>> +	 * (e.g. i915_fence_get_driver_name). We could likely change these
>>>> +	 * functions to avoid touching the engine but let's just be safe and
>>>> +	 * hold the intel_context reference.
>>>> +	 */
>>>> +	rq->context = intel_context_get(ce);
>>>>    	rq->engine = ce->engine;
>>>>    	rq->ring = ce->ring;
>>>>    	rq->execution_mask = ce->engine->mask;
>>>> @@ -1054,6 +1043,7 @@ __i915_request_create(struct intel_context *ce, gfp_t gfp)
>>>>    	GEM_BUG_ON(!list_empty(&rq->sched.waiters_list));
>>>>    err_free:
>>>> +	intel_context_put(ce);
>>>>    	kmem_cache_free(global.slab_requests, rq);
>>>>    err_unreserve:
>>>>    	intel_context_unpin(ce);
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