On Wed, Jun 15, 2022 at 12:13:47AM +0530, Ramalingam C wrote:
> From: Niranjana Vishwanathapura <niranjana.vishwanathapura@xxxxxxxxx>
>
> This reverts commit 1e98d8c52ed5dfbaf273c4423c636525c2ce59e7.
>
> The problem with this patch is that it makes i915_request to hold a
> reference to intel_context, which in turn holds a reference on the VM.
> This strong back referencing can lead to reference loops which leads
> to resource leak.
>
> An example is the upcoming VM_BIND work which requires VM to hold
> a reference to some shared VM specific BO. But this BO's dma-resv
> fences holds reference to the i915_request thus leading to reference
> loop.
>
> Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@xxxxxxxxx>
> Signed-off-by: Ramalingam C <ramalingam.c@xxxxxxxxx>
> Suggested-by: Matthew Brost <matthew.brost@xxxxxxxxx>
Talked with Ram, this patch needs to be squashed with the following
patch. The reasoning is with just this patch, the tree is broken -
parallel submission contexts will leak requests.
With the patches squashed:
Reviewed-by: Mathew Brost <matthew.brost@xxxxxxxxx>
> ---
> drivers/gpu/drm/i915/i915_request.c | 55 +++++++++++++++++------------
> 1 file changed, 32 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index 7f6998bf390c..c71905d8e154 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -134,17 +134,39 @@ 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,
> - * do not use with virtual engines as this really is only needed for
> - * kernel contexts.
> + * 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.
> */
> - if (!intel_engine_is_virtual(rq->engine) &&
> - !cmpxchg(&rq->engine->request_pool, NULL, rq)) {
> - intel_context_put(rq->context);
> + if (is_power_of_2(rq->execution_mask) &&
> + !cmpxchg(&rq->engine->request_pool, NULL, rq))
> return;
> - }
> -
> - intel_context_put(rq->context);
>
> kmem_cache_free(slab_requests, rq);
> }
> @@ -921,19 +943,7 @@ __i915_request_create(struct intel_context *ce, gfp_t gfp)
> }
> }
>
> - /*
> - * 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. In execlist mode the request always
> - * eventually points to a physical engine so this isn't an issue.
> - */
> - rq->context = intel_context_get(ce);
> + rq->context = ce;
> rq->engine = ce->engine;
> rq->ring = ce->ring;
> rq->execution_mask = ce->engine->mask;
> @@ -1009,7 +1019,6 @@ __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(slab_requests, rq);
> err_unreserve:
> intel_context_unpin(ce);
> --
> 2.20.1
>
