[Intel-gfx] [PATCH 03/33] drm/i915: Hold reference to intel_context over life of i915_request

Matthew Brost
2021-07-22T23:37:10Z

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.

v2:
(John Harrison)
- Update comment explaining how GuC mode and execlists mode deal with
virtual engines differently

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
---
drivers/gpu/drm/i915/i915_request.c | 55 ++++++++++++-----------------
1 file changed, 23 insertions(+), 32 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 39a21d96577e..57c9187aff74 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -125,39 +125,17 @@ 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);
}
@@ -956,7 +934,19 @@ __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. 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->engine = ce->engine;
rq->ring = ce->ring;
rq->execution_mask = ce->engine->mask;
@@ -1033,6 +1023,7 @@ __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);