drm/i915/gt: Free stale request on destroying the virtual engine

Since preempt-to-busy, we may unsubmit a request while it is still on
the HW and completes asynchronously. That means it may be retired and in
the process destroy the virtual engine (as the user has closed their
context), but that engine may still be holding onto the unsubmitted
compelted request. Therefore we need to potentially cleanup the old
request on destroying the virtual engine. We also have to keep the
virtual_engine alive until after the sibling's execlists_dequeue() have
finished peeking into the virtual engines, for which we serialise with
RCU.

v2: Be paranoid and flush the tasklet as well.
v3: And flush the tasklet before the engines, as the tasklet may
re-attach an rb_node after our removal from the siblings.

Fixes: 6d06779 ("drm/i915: Load balancing across a virtual engine")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201123113717.20500-4-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_lrc.c

@@ -182,6 +182,7 @@
 struct virtual_engine {
 	struct intel_engine_cs base;
 	struct intel_context context;
+	struct rcu_work rcu;
 
 	/*
 	 * We allow only a single request through the virtual engine at a time
@@ -5470,33 +5471,57 @@ static struct list_head *virtual_queue(struct virtual_engine *ve)
 	return &ve->base.execlists.default_priolist.requests[0];
 }
 
-static void virtual_context_destroy(struct kref *kref)
+static void rcu_virtual_context_destroy(struct work_struct *wrk)
 {
 	struct virtual_engine *ve =
-		container_of(kref, typeof(*ve), context.ref);
+		container_of(wrk, typeof(*ve), rcu.work);
 	unsigned int n;
 
-	GEM_BUG_ON(!list_empty(virtual_queue(ve)));
-	GEM_BUG_ON(ve->request);
 	GEM_BUG_ON(ve->context.inflight);
 
+	/* Preempt-to-busy may leave a stale request behind. */
+	if (unlikely(ve->request)) {
+		struct i915_request *old;
+
+		spin_lock_irq(&ve->base.active.lock);
+
+		old = fetch_and_zero(&ve->request);
+		if (old) {
+			GEM_BUG_ON(!i915_request_completed(old));
+			__i915_request_submit(old);
+			i915_request_put(old);
+		}
+
+		spin_unlock_irq(&ve->base.active.lock);
+	}
+
+	/*
+	 * Flush the tasklet in case it is still running on another core.
+	 *
+	 * This needs to be done before we remove ourselves from the siblings'
+	 * rbtrees as in the case it is running in parallel, it may reinsert
+	 * the rb_node into a sibling.
+	 */
+	tasklet_kill(&ve->base.execlists.tasklet);
+
+	/* Decouple ourselves from the siblings, no more access allowed. */
 	for (n = 0; n < ve->num_siblings; n++) {
 		struct intel_engine_cs *sibling = ve->siblings[n];
 		struct rb_node *node = &ve->nodes[sibling->id].rb;
-		unsigned long flags;
 
 		if (RB_EMPTY_NODE(node))
 			continue;
 
-		spin_lock_irqsave(&sibling->active.lock, flags);
+		spin_lock_irq(&sibling->active.lock);
 
 		/* Detachment is lazily performed in the execlists tasklet */
 		if (!RB_EMPTY_NODE(node))
 			rb_erase_cached(node, &sibling->execlists.virtual);
 
-		spin_unlock_irqrestore(&sibling->active.lock, flags);
+		spin_unlock_irq(&sibling->active.lock);
 	}
 	GEM_BUG_ON(__tasklet_is_scheduled(&ve->base.execlists.tasklet));
+	GEM_BUG_ON(!list_empty(virtual_queue(ve)));
 
 	if (ve->context.state)
 		__execlists_context_fini(&ve->context);
@@ -5509,6 +5534,27 @@ static void virtual_context_destroy(struct kref *kref)
 	kfree(ve);
 }
 
+static void virtual_context_destroy(struct kref *kref)
+{
+	struct virtual_engine *ve =
+		container_of(kref, typeof(*ve), context.ref);
+
+	GEM_BUG_ON(!list_empty(&ve->context.signals));
+
+	/*
+	 * When destroying the virtual engine, we have to be aware that
+	 * it may still be in use from an hardirq/softirq context causing
+	 * the resubmission of a completed request (background completion
+	 * due to preempt-to-busy). Before we can free the engine, we need
+	 * to flush the submission code and tasklets that are still potentially
+	 * accessing the engine. Flushing the tasklets requires process context,
+	 * and since we can guard the resubmit onto the engine with an RCU read
+	 * lock, we can delegate the free of the engine to an RCU worker.
+	 */
+	INIT_RCU_WORK(&ve->rcu, rcu_virtual_context_destroy);
+	queue_rcu_work(system_wq, &ve->rcu);
+}
+
 static void virtual_engine_initial_hint(struct virtual_engine *ve)
 {
 	int swp;