On 06/05/2021 20:14, 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. 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.
Isn't this a bug in present upstream as well? Like calling sync fence info on retired requests or something else?
If it is a bug in upstream then I think a single patch to deal with the issue should be posted independently. It may be as simple as checking for the signaled bit in i915_fence_get_driver_name and dereferencing with rcu protection.
Regards, Tvrtko
Signed-off-by: Matthew Brost <matthew.br...@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 127d60b36422..0b96b824ea06 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);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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