drm/xe: Make dma-fences compliant with the safe access rules

Xe can free some of the data pointed to by the dma-fences it exports. Most
notably the timeline name can get freed if userspace closes the associated
submit queue. At the same time the fence could have been exported to a
third party (for example a sync_fence fd) which will then cause an use-
after-free on subsequent access.

To make this safe we need to make the driver compliant with the newly
documented dma-fence rules. Driver has to ensure a RCU grace period
between signalling a fence and freeing any data pointed to by said fence.

For the timeline name we simply make the queue be freed via kfree_rcu and
for the shared lock associated with multiple queues we add a RCU grace
period before freeing the per GT structure holding the lock.

Signed-off-by: Tvrtko Ursulin <[e-mail]>
Reviewed-by: Matthew Brost <[e-mail]>
Acked-by: Lucas De Marchi <[e-mail]>
Signed-off-by: Tvrtko Ursulin <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/gpu/drm/xe/xe_guc_exec_queue_types.h

···
 struct xe_guc_exec_queue {
 	/** @q: Backpointer to parent xe_exec_queue */
 	struct xe_exec_queue *q;
+	/** @rcu: For safe freeing of exported dma fences */
+	struct rcu_head rcu;
 	/** @sched: GPU scheduler for this xe_exec_queue */
 	struct xe_gpu_scheduler sched;
 	/** @entity: Scheduler entity for this xe_exec_queue */

drivers/gpu/drm/xe/xe_guc_submit.c

···
 	xe_sched_entity_fini(&ge->entity);
 	xe_sched_fini(&ge->sched);
 
-	kfree(ge);
+	/*
+	 * RCU free due sched being exported via DRM scheduler fences
+	 * (timeline name).
+	 */
+	kfree_rcu(ge, rcu);
 	xe_exec_queue_fini(q);
 	xe_pm_runtime_put(guc_to_xe(guc));
 }
···
 
 	q->guc = ge;
 	ge->q = q;
+	init_rcu_head(&ge->rcu);
 	init_waitqueue_head(&ge->suspend_wait);
 
 	for (i = 0; i < MAX_STATIC_MSG_TYPE; ++i)

drivers/gpu/drm/xe/xe_hw_fence.c

···
 		spin_unlock_irqrestore(&irq->lock, flags);
 		dma_fence_end_signalling(tmp);
 	}
+
+	/* Safe release of the irq->lock used in dma_fence_init. */
+	synchronize_rcu();
 }
 
 void xe_hw_fence_irq_run(struct xe_hw_fence_irq *irq)