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>
---
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
