Revert "drm/i915: Hold reference to intel_context over life of i915_request"

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.

v2:
  Do not use reserved requests for virtual engines

Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Suggested-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Mathew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220614184348.23746-3-ramalingam.c@intel.com

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 667dda7668cb1bb81f12622be5e4b5995a4abb37..62fad16a55e84f15ba3784a298e1155349f64297 100644 (file)
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -134,17 +134,42 @@ 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,
+        * 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.
+        *
+        * 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);
+           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 +946,7 @@ __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 +1022,6 @@ err_unwind:
        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);