1 files changed, 642 insertions, 177 deletions

diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
index 293e5bcc4b6c..b0a499753526 100644
--- a/drivers/gpu/drm/i915/i915_active.c
+++ b/drivers/gpu/drm/i915/i915_active.c
@@ -4,14 +4,16 @@
  * Copyright © 2019 Intel Corporation
  */
 
+#include <linux/debugobjects.h>
+
+#include "gt/intel_context.h"
 #include "gt/intel_engine_pm.h"
+#include "gt/intel_ring.h"
 
 #include "i915_drv.h"
 #include "i915_active.h"
 #include "i915_globals.h"
 
-#define BKL(ref) (&(ref)->i915->drm.struct_mutex)
-
 /*
  * Active refs memory management
  *
@@ -25,55 +27,200 @@ static struct i915_global_active {
 } global;
 
 struct active_node {
-	struct i915_active_request base;
+	struct i915_active_fence base;
 	struct i915_active *ref;
 	struct rb_node node;
 	u64 timeline;
 };
 
+static inline struct active_node *
+node_from_active(struct i915_active_fence *active)
+{
+	return container_of(active, struct active_node, base);
+}
+
+#define take_preallocated_barriers(x) llist_del_all(&(x)->preallocated_barriers)
+
+static inline bool is_barrier(const struct i915_active_fence *active)
+{
+	return IS_ERR(rcu_access_pointer(active->fence));
+}
+
+static inline struct llist_node *barrier_to_ll(struct active_node *node)
+{
+	GEM_BUG_ON(!is_barrier(&node->base));
+	return (struct llist_node *)&node->base.cb.node;
+}
+
+static inline struct intel_engine_cs *
+__barrier_to_engine(struct active_node *node)
+{
+	return (struct intel_engine_cs *)READ_ONCE(node->base.cb.node.prev);
+}
+
+static inline struct intel_engine_cs *
+barrier_to_engine(struct active_node *node)
+{
+	GEM_BUG_ON(!is_barrier(&node->base));
+	return __barrier_to_engine(node);
+}
+
+static inline struct active_node *barrier_from_ll(struct llist_node *x)
+{
+	return container_of((struct list_head *)x,
+			    struct active_node, base.cb.node);
+}
+
+#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM) && IS_ENABLED(CONFIG_DEBUG_OBJECTS)
+
+static void *active_debug_hint(void *addr)
+{
+	struct i915_active *ref = addr;
+
+	return (void *)ref->active ?: (void *)ref->retire ?: (void *)ref;
+}
+
+static struct debug_obj_descr active_debug_desc = {
+	.name = "i915_active",
+	.debug_hint = active_debug_hint,
+};
+
+static void debug_active_init(struct i915_active *ref)
+{
+	debug_object_init(ref, &active_debug_desc);
+}
+
+static void debug_active_activate(struct i915_active *ref)
+{
+	lockdep_assert_held(&ref->tree_lock);
+	if (!atomic_read(&ref->count)) /* before the first inc */
+		debug_object_activate(ref, &active_debug_desc);
+}
+
+static void debug_active_deactivate(struct i915_active *ref)
+{
+	lockdep_assert_held(&ref->tree_lock);
+	if (!atomic_read(&ref->count)) /* after the last dec */
+		debug_object_deactivate(ref, &active_debug_desc);
+}
+
+static void debug_active_fini(struct i915_active *ref)
+{
+	debug_object_free(ref, &active_debug_desc);
+}
+
+static void debug_active_assert(struct i915_active *ref)
+{
+	debug_object_assert_init(ref, &active_debug_desc);
+}
+
+#else
+
+static inline void debug_active_init(struct i915_active *ref) { }
+static inline void debug_active_activate(struct i915_active *ref) { }
+static inline void debug_active_deactivate(struct i915_active *ref) { }
+static inline void debug_active_fini(struct i915_active *ref) { }
+static inline void debug_active_assert(struct i915_active *ref) { }
+
+#endif
+
 static void
-__active_park(struct i915_active *ref)
+__active_retire(struct i915_active *ref)
 {
 	struct active_node *it, *n;
+	struct rb_root root;
+	unsigned long flags;
 
-	rbtree_postorder_for_each_entry_safe(it, n, &ref->tree, node) {
-		GEM_BUG_ON(i915_active_request_isset(&it->base));
+	GEM_BUG_ON(i915_active_is_idle(ref));
+
+	/* return the unused nodes to our slabcache -- flushing the allocator */
+	if (!atomic_dec_and_lock_irqsave(&ref->count, &ref->tree_lock, flags))
+		return;
+
+	GEM_BUG_ON(rcu_access_pointer(ref->excl.fence));
+	debug_active_deactivate(ref);
+
+	root = ref->tree;
+	ref->tree = RB_ROOT;
+	ref->cache = NULL;
+
+	spin_unlock_irqrestore(&ref->tree_lock, flags);
+
+	/* After the final retire, the entire struct may be freed */
+	if (ref->retire)
+		ref->retire(ref);
+
+	/* ... except if you wait on it, you must manage your own references! */
+	wake_up_var(ref);
+
+	rbtree_postorder_for_each_entry_safe(it, n, &root, node) {
+		GEM_BUG_ON(i915_active_fence_isset(&it->base));
 		kmem_cache_free(global.slab_cache, it);
 	}
-	ref->tree = RB_ROOT;
 }
 
 static void
-__active_retire(struct i915_active *ref)
+active_work(struct work_struct *wrk)
 {
-	GEM_BUG_ON(!ref->count);
-	if (--ref->count)
+	struct i915_active *ref = container_of(wrk, typeof(*ref), work);
+
+	GEM_BUG_ON(!atomic_read(&ref->count));
+	if (atomic_add_unless(&ref->count, -1, 1))
 		return;
 
-	/* return the unused nodes to our slabcache */
-	__active_park(ref);
+	__active_retire(ref);
+}
+
+static void
+active_retire(struct i915_active *ref)
+{
+	GEM_BUG_ON(!atomic_read(&ref->count));
+	if (atomic_add_unless(&ref->count, -1, 1))
+		return;
+
+	if (ref->flags & I915_ACTIVE_RETIRE_SLEEPS) {
+		queue_work(system_unbound_wq, &ref->work);
+		return;
+	}
+
+	__active_retire(ref);
+}
+
+static inline struct dma_fence **
+__active_fence_slot(struct i915_active_fence *active)
+{
+	return (struct dma_fence ** __force)&active->fence;
+}
 
-	ref->retire(ref);
+static inline bool
+active_fence_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
+{
+	struct i915_active_fence *active =
+		container_of(cb, typeof(*active), cb);
+
+	return cmpxchg(__active_fence_slot(active), fence, NULL) == fence;
 }
 
 static void
-node_retire(struct i915_active_request *base, struct i915_request *rq)
+node_retire(struct dma_fence *fence, struct dma_fence_cb *cb)
 {
-	__active_retire(container_of(base, struct active_node, base)->ref);
+	if (active_fence_cb(fence, cb))
+		active_retire(container_of(cb, struct active_node, base.cb)->ref);
 }
 
 static void
-last_retire(struct i915_active_request *base, struct i915_request *rq)
+excl_retire(struct dma_fence *fence, struct dma_fence_cb *cb)
 {
-	__active_retire(container_of(base, struct i915_active, last));
+	if (active_fence_cb(fence, cb))
+		active_retire(container_of(cb, struct i915_active, excl.cb));
 }
 
-static struct i915_active_request *
-active_instance(struct i915_active *ref, u64 idx)
+static struct i915_active_fence *
+active_instance(struct i915_active *ref, struct intel_timeline *tl)
 {
-	struct active_node *node;
+	struct active_node *node, *prealloc;
 	struct rb_node **p, *parent;
-	struct i915_request *old;
+	u64 idx = tl->fence_context;
 
 	/*
 	 * We track the most recently used timeline to skip a rbtree search
@@ -81,20 +228,18 @@ active_instance(struct i915_active *ref, u64 idx)
 	 * at all. We can reuse the last slot if it is empty, that is
 	 * after the previous activity has been retired, or if it matches the
 	 * current timeline.
-	 *
-	 * Note that we allow the timeline to be active simultaneously in
-	 * the rbtree and the last cache. We do this to avoid having
-	 * to search and replace the rbtree element for a new timeline, with
-	 * the cost being that we must be aware that the ref may be retired
-	 * twice for the same timeline (as the older rbtree element will be
-	 * retired before the new request added to last).
 	 */
-	old = i915_active_request_raw(&ref->last, BKL(ref));
-	if (!old || old->fence.context == idx)
-		goto out;
+	node = READ_ONCE(ref->cache);
+	if (node && node->timeline == idx)
+		return &node->base;
 
-	/* Move the currently active fence into the rbtree */
-	idx = old->fence.context;
+	/* Preallocate a replacement, just in case */
+	prealloc = kmem_cache_alloc(global.slab_cache, GFP_KERNEL);
+	if (!prealloc)
+		return NULL;
+
+	spin_lock_irq(&ref->tree_lock);
+	GEM_BUG_ON(i915_active_is_idle(ref));
 
 	parent = NULL;
 	p = &ref->tree.rb_node;
@@ -102,8 +247,10 @@ active_instance(struct i915_active *ref, u64 idx)
 		parent = *p;
 
 		node = rb_entry(parent, struct active_node, node);
-		if (node->timeline == idx)
-			goto replace;
+		if (node->timeline == idx) {
+			kmem_cache_free(global.slab_cache, prealloc);
+			goto out;
+		}
 
 		if (node->timeline < idx)
 			p = &parent->rb_right;
@@ -111,202 +258,428 @@ active_instance(struct i915_active *ref, u64 idx)
 			p = &parent->rb_left;
 	}
 
-	node = kmem_cache_alloc(global.slab_cache, GFP_KERNEL);
-
-	/* kmalloc may retire the ref->last (thanks shrinker)! */
-	if (unlikely(!i915_active_request_raw(&ref->last, BKL(ref)))) {
-		kmem_cache_free(global.slab_cache, node);
-		goto out;
-	}
-
-	if (unlikely(!node))
-		return ERR_PTR(-ENOMEM);
-
-	i915_active_request_init(&node->base, NULL, node_retire);
+	node = prealloc;
+	__i915_active_fence_init(&node->base, NULL, node_retire);
 	node->ref = ref;
 	node->timeline = idx;
 
 	rb_link_node(&node->node, parent, p);
 	rb_insert_color(&node->node, &ref->tree);
 
-replace:
+out:
+	ref->cache = node;
+	spin_unlock_irq(&ref->tree_lock);
+
+	BUILD_BUG_ON(offsetof(typeof(*node), base));
+	return &node->base;
+}
+
+void __i915_active_init(struct i915_active *ref,
+			int (*active)(struct i915_active *ref),
+			void (*retire)(struct i915_active *ref),
+			struct lock_class_key *mkey,
+			struct lock_class_key *wkey)
+{
+	unsigned long bits;
+
+	debug_active_init(ref);
+
+	ref->flags = 0;
+	ref->active = active;
+	ref->retire = ptr_unpack_bits(retire, &bits, 2);
+	if (bits & I915_ACTIVE_MAY_SLEEP)
+		ref->flags |= I915_ACTIVE_RETIRE_SLEEPS;
+
+	spin_lock_init(&ref->tree_lock);
+	ref->tree = RB_ROOT;
+	ref->cache = NULL;
+
+	init_llist_head(&ref->preallocated_barriers);
+	atomic_set(&ref->count, 0);
+	__mutex_init(&ref->mutex, "i915_active", mkey);
+	__i915_active_fence_init(&ref->excl, NULL, excl_retire);
+	INIT_WORK(&ref->work, active_work);
+#if IS_ENABLED(CONFIG_LOCKDEP)
+	lockdep_init_map(&ref->work.lockdep_map, "i915_active.work", wkey, 0);
+#endif
+}
+
+static bool ____active_del_barrier(struct i915_active *ref,
+				   struct active_node *node,
+				   struct intel_engine_cs *engine)
+
+{
+	struct llist_node *head = NULL, *tail = NULL;
+	struct llist_node *pos, *next;
+
+	GEM_BUG_ON(node->timeline != engine->kernel_context->timeline->fence_context);
+
 	/*
-	 * Overwrite the previous active slot in the rbtree with last,
-	 * leaving last zeroed. If the previous slot is still active,
-	 * we must be careful as we now only expect to receive one retire
-	 * callback not two, and so much undo the active counting for the
-	 * overwritten slot.
+	 * Rebuild the llist excluding our node. We may perform this
+	 * outside of the kernel_context timeline mutex and so someone
+	 * else may be manipulating the engine->barrier_tasks, in
+	 * which case either we or they will be upset :)
+	 *
+	 * A second __active_del_barrier() will report failure to claim
+	 * the active_node and the caller will just shrug and know not to
+	 * claim ownership of its node.
+	 *
+	 * A concurrent i915_request_add_active_barriers() will miss adding
+	 * any of the tasks, but we will try again on the next -- and since
+	 * we are actively using the barrier, we know that there will be
+	 * at least another opportunity when we idle.
 	 */
-	if (i915_active_request_isset(&node->base)) {
-		/* Retire ourselves from the old rq->active_list */
-		__list_del_entry(&node->base.link);
-		ref->count--;
-		GEM_BUG_ON(!ref->count);
+	llist_for_each_safe(pos, next, llist_del_all(&engine->barrier_tasks)) {
+		if (node == barrier_from_ll(pos)) {
+			node = NULL;
+			continue;
+		}
+
+		pos->next = head;
+		head = pos;
+		if (!tail)
+			tail = pos;
 	}
-	GEM_BUG_ON(list_empty(&ref->last.link));
-	list_replace_init(&ref->last.link, &node->base.link);
-	node->base.request = fetch_and_zero(&ref->last.request);
+	if (head)
+		llist_add_batch(head, tail, &engine->barrier_tasks);
 
-out:
-	return &ref->last;
+	return !node;
 }
 
-void i915_active_init(struct drm_i915_private *i915,
-		      struct i915_active *ref,
-		      void (*retire)(struct i915_active *ref))
+static bool
+__active_del_barrier(struct i915_active *ref, struct active_node *node)
 {
-	ref->i915 = i915;
-	ref->retire = retire;
-	ref->tree = RB_ROOT;
-	i915_active_request_init(&ref->last, NULL, last_retire);
-	init_llist_head(&ref->barriers);
-	ref->count = 0;
+	return ____active_del_barrier(ref, node, barrier_to_engine(node));
 }
 
 int i915_active_ref(struct i915_active *ref,
-		    u64 timeline,
-		    struct i915_request *rq)
+		    struct intel_timeline *tl,
+		    struct dma_fence *fence)
 {
-	struct i915_active_request *active;
-	int err = 0;
+	struct i915_active_fence *active;
+	int err;
+
+	lockdep_assert_held(&tl->mutex);
 
 	/* Prevent reaping in case we malloc/wait while building the tree */
-	i915_active_acquire(ref);
+	err = i915_active_acquire(ref);
+	if (err)
+		return err;
 
-	active = active_instance(ref, timeline);
-	if (IS_ERR(active)) {
-		err = PTR_ERR(active);
+	active = active_instance(ref, tl);
+	if (!active) {
+		err = -ENOMEM;
 		goto out;
 	}
 
-	if (!i915_active_request_isset(active))
-		ref->count++;
-	__i915_active_request_set(active, rq);
+	if (is_barrier(active)) { /* proto-node used by our idle barrier */
+		/*
+		 * This request is on the kernel_context timeline, and so
+		 * we can use it to substitute for the pending idle-barrer
+		 * request that we want to emit on the kernel_context.
+		 */
+		__active_del_barrier(ref, node_from_active(active));
+		RCU_INIT_POINTER(active->fence, NULL);
+		atomic_dec(&ref->count);
+	}
+	if (!__i915_active_fence_set(active, fence))
+		atomic_inc(&ref->count);
 
-	GEM_BUG_ON(!ref->count);
 out:
 	i915_active_release(ref);
 	return err;
 }
 
-bool i915_active_acquire(struct i915_active *ref)
+void i915_active_set_exclusive(struct i915_active *ref, struct dma_fence *f)
 {
-	lockdep_assert_held(BKL(ref));
-	return !ref->count++;
+	/* We expect the caller to manage the exclusive timeline ordering */
+	GEM_BUG_ON(i915_active_is_idle(ref));
+
+	if (!__i915_active_fence_set(&ref->excl, f))
+		atomic_inc(&ref->count);
 }
 
-void i915_active_release(struct i915_active *ref)
+bool i915_active_acquire_if_busy(struct i915_active *ref)
 {
-	lockdep_assert_held(BKL(ref));
-	__active_retire(ref);
+	debug_active_assert(ref);
+	return atomic_add_unless(&ref->count, 1, 0);
 }
 
-int i915_active_wait(struct i915_active *ref)
+int i915_active_acquire(struct i915_active *ref)
 {
-	struct active_node *it, *n;
-	int ret = 0;
+	int err;
 
-	if (i915_active_acquire(ref))
-		goto out_release;
+	if (i915_active_acquire_if_busy(ref))
+		return 0;
 
-	ret = i915_active_request_retire(&ref->last, BKL(ref));
-	if (ret)
-		goto out_release;
+	err = mutex_lock_interruptible(&ref->mutex);
+	if (err)
+		return err;
 
-	rbtree_postorder_for_each_entry_safe(it, n, &ref->tree, node) {
-		ret = i915_active_request_retire(&it->base, BKL(ref));
-		if (ret)
-			break;
+	if (likely(!i915_active_acquire_if_busy(ref))) {
+		if (ref->active)
+			err = ref->active(ref);
+		if (!err) {
+			spin_lock_irq(&ref->tree_lock); /* __active_retire() */
+			debug_active_activate(ref);
+			atomic_inc(&ref->count);
+			spin_unlock_irq(&ref->tree_lock);
+		}
 	}
 
-out_release:
-	i915_active_release(ref);
-	return ret;
+	mutex_unlock(&ref->mutex);
+
+	return err;
+}
+
+void i915_active_release(struct i915_active *ref)
+{
+	debug_active_assert(ref);
+	active_retire(ref);
 }
 
-int i915_request_await_active_request(struct i915_request *rq,
-				      struct i915_active_request *active)
+static void enable_signaling(struct i915_active_fence *active)
 {
-	struct i915_request *barrier =
-		i915_active_request_raw(active, &rq->i915->drm.struct_mutex);
+	struct dma_fence *fence;
 
-	return barrier ? i915_request_await_dma_fence(rq, &barrier->fence) : 0;
+	fence = i915_active_fence_get(active);
+	if (!fence)
+		return;
+
+	dma_fence_enable_sw_signaling(fence);
+	dma_fence_put(fence);
 }
 
-int i915_request_await_active(struct i915_request *rq, struct i915_active *ref)
+int i915_active_wait(struct i915_active *ref)
 {
 	struct active_node *it, *n;
 	int err = 0;
 
-	/* await allocates and so we need to avoid hitting the shrinker */
-	if (i915_active_acquire(ref))
-		goto out; /* was idle */
+	might_sleep();
 
-	err = i915_request_await_active_request(rq, &ref->last);
-	if (err)
-		goto out;
+	if (!i915_active_acquire_if_busy(ref))
+		return 0;
 
+	/* Flush lazy signals */
+	enable_signaling(&ref->excl);
 	rbtree_postorder_for_each_entry_safe(it, n, &ref->tree, node) {
-		err = i915_request_await_active_request(rq, &it->base);
-		if (err)
-			goto out;
+		if (is_barrier(&it->base)) /* unconnected idle barrier */
+			continue;
+
+		enable_signaling(&it->base);
 	}
+	/* Any fence added after the wait begins will not be auto-signaled */
 
-out:
 	i915_active_release(ref);
+	if (err)
+		return err;
+
+	if (wait_var_event_interruptible(ref, i915_active_is_idle(ref)))
+		return -EINTR;
+
+	flush_work(&ref->work);
+	return 0;
+}
+
+int i915_request_await_active(struct i915_request *rq, struct i915_active *ref)
+{
+	int err = 0;
+
+	if (rcu_access_pointer(ref->excl.fence)) {
+		struct dma_fence *fence;
+
+		rcu_read_lock();
+		fence = dma_fence_get_rcu_safe(&ref->excl.fence);
+		rcu_read_unlock();
+		if (fence) {
+			err = i915_request_await_dma_fence(rq, fence);
+			dma_fence_put(fence);
+		}
+	}
+
+	/* In the future we may choose to await on all fences */
+
 	return err;
 }
 
 #if IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM)
 void i915_active_fini(struct i915_active *ref)
 {
-	GEM_BUG_ON(i915_active_request_isset(&ref->last));
+	debug_active_fini(ref);
+	GEM_BUG_ON(atomic_read(&ref->count));
+	GEM_BUG_ON(work_pending(&ref->work));
 	GEM_BUG_ON(!RB_EMPTY_ROOT(&ref->tree));
-	GEM_BUG_ON(ref->count);
+	mutex_destroy(&ref->mutex);
 }
 #endif
 
+static inline bool is_idle_barrier(struct active_node *node, u64 idx)
+{
+	return node->timeline == idx && !i915_active_fence_isset(&node->base);
+}
+
+static struct active_node *reuse_idle_barrier(struct i915_active *ref, u64 idx)
+{
+	struct rb_node *prev, *p;
+
+	if (RB_EMPTY_ROOT(&ref->tree))
+		return NULL;
+
+	spin_lock_irq(&ref->tree_lock);
+	GEM_BUG_ON(i915_active_is_idle(ref));
+
+	/*
+	 * Try to reuse any existing barrier nodes already allocated for this
+	 * i915_active, due to overlapping active phases there is likely a
+	 * node kept alive (as we reuse before parking). We prefer to reuse
+	 * completely idle barriers (less hassle in manipulating the llists),
+	 * but otherwise any will do.
+	 */
+	if (ref->cache && is_idle_barrier(ref->cache, idx)) {
+		p = &ref->cache->node;
+		goto match;
+	}
+
+	prev = NULL;
+	p = ref->tree.rb_node;
+	while (p) {
+		struct active_node *node =
+			rb_entry(p, struct active_node, node);
+
+		if (is_idle_barrier(node, idx))
+			goto match;
+
+		prev = p;
+		if (node->timeline < idx)
+			p = p->rb_right;
+		else
+			p = p->rb_left;
+	}
+
+	/*
+	 * No quick match, but we did find the leftmost rb_node for the
+	 * kernel_context. Walk the rb_tree in-order to see if there were
+	 * any idle-barriers on this timeline that we missed, or just use
+	 * the first pending barrier.
+	 */
+	for (p = prev; p; p = rb_next(p)) {
+		struct active_node *node =
+			rb_entry(p, struct active_node, node);
+		struct intel_engine_cs *engine;
+
+		if (node->timeline > idx)
+			break;
+
+		if (node->timeline < idx)
+			continue;
+
+		if (is_idle_barrier(node, idx))
+			goto match;
+
+		/*
+		 * The list of pending barriers is protected by the
+		 * kernel_context timeline, which notably we do not hold
+		 * here. i915_request_add_active_barriers() may consume
+		 * the barrier before we claim it, so we have to check
+		 * for success.
+		 */
+		engine = __barrier_to_engine(node);
+		smp_rmb(); /* serialise with add_active_barriers */
+		if (is_barrier(&node->base) &&
+		    ____active_del_barrier(ref, node, engine))
+			goto match;
+	}
+
+	spin_unlock_irq(&ref->tree_lock);
+
+	return NULL;
+
+match:
+	rb_erase(p, &ref->tree); /* Hide from waits and sibling allocations */
+	if (p == &ref->cache->node)
+		ref->cache = NULL;
+	spin_unlock_irq(&ref->tree_lock);
+
+	return rb_entry(p, struct active_node, node);
+}
+
 int i915_active_acquire_preallocate_barrier(struct i915_active *ref,
 					    struct intel_engine_cs *engine)
 {
-	struct drm_i915_private *i915 = engine->i915;
-	struct llist_node *pos, *next;
-	unsigned long tmp;
+	intel_engine_mask_t tmp, mask = engine->mask;
+	struct llist_node *first = NULL, *last = NULL;
+	struct intel_gt *gt = engine->gt;
 	int err;
 
-	GEM_BUG_ON(!engine->mask);
-	for_each_engine_masked(engine, i915, engine->mask, tmp) {
-		struct intel_context *kctx = engine->kernel_context;
+	GEM_BUG_ON(i915_active_is_idle(ref));
+
+	/* Wait until the previous preallocation is completed */
+	while (!llist_empty(&ref->preallocated_barriers))
+		cond_resched();
+
+	/*
+	 * Preallocate a node for each physical engine supporting the target
+	 * engine (remember virtual engines have more than one sibling).
+	 * We can then use the preallocated nodes in
+	 * i915_active_acquire_barrier()
+	 */
+	for_each_engine_masked(engine, gt, mask, tmp) {
+		u64 idx = engine->kernel_context->timeline->fence_context;
+		struct llist_node *prev = first;
 		struct active_node *node;
 
-		node = kmem_cache_alloc(global.slab_cache, GFP_KERNEL);
-		if (unlikely(!node)) {
-			err = -ENOMEM;
-			goto unwind;
+		node = reuse_idle_barrier(ref, idx);
+		if (!node) {
+			node = kmem_cache_alloc(global.slab_cache, GFP_KERNEL);
+			if (!node) {
+				err = ENOMEM;
+				goto unwind;
+			}
+
+			RCU_INIT_POINTER(node->base.fence, NULL);
+			node->base.cb.func = node_retire;
+			node->timeline = idx;
+			node->ref = ref;
 		}
 
-		i915_active_request_init(&node->base,
-					 (void *)engine, node_retire);
-		node->timeline = kctx->ring->timeline->fence_context;
-		node->ref = ref;
-		ref->count++;
+		if (!i915_active_fence_isset(&node->base)) {
+			/*
+			 * Mark this as being *our* unconnected proto-node.
+			 *
+			 * Since this node is not in any list, and we have
+			 * decoupled it from the rbtree, we can reuse the
+			 * request to indicate this is an idle-barrier node
+			 * and then we can use the rb_node and list pointers
+			 * for our tracking of the pending barrier.
+			 */
+			RCU_INIT_POINTER(node->base.fence, ERR_PTR(-EAGAIN));
+			node->base.cb.node.prev = (void *)engine;
+			atomic_inc(&ref->count);
+		}
+		GEM_BUG_ON(rcu_access_pointer(node->base.fence) != ERR_PTR(-EAGAIN));
 
+		GEM_BUG_ON(barrier_to_engine(node) != engine);
+		first = barrier_to_ll(node);
+		first->next = prev;
+		if (!last)
+			last = first;
 		intel_engine_pm_get(engine);
-		llist_add((struct llist_node *)&node->base.link,
-			  &ref->barriers);
 	}
 
+	GEM_BUG_ON(!llist_empty(&ref->preallocated_barriers));
+	llist_add_batch(first, last, &ref->preallocated_barriers);
+
 	return 0;
 
 unwind:
-	llist_for_each_safe(pos, next, llist_del_all(&ref->barriers)) {
-		struct active_node *node;
+	while (first) {
+		struct active_node *node = barrier_from_ll(first);
 
-		node = container_of((struct list_head *)pos,
-				    typeof(*node), base.link);
-		engine = (void *)rcu_access_pointer(node->base.request);
+		first = first->next;
+
+		atomic_dec(&ref->count);
+		intel_engine_pm_put(barrier_to_engine(node));
 
-		intel_engine_pm_put(engine);
 		kmem_cache_free(global.slab_cache, node);
 	}
 	return err;
@@ -315,68 +688,160 @@ unwind:
 void i915_active_acquire_barrier(struct i915_active *ref)
 {
 	struct llist_node *pos, *next;
+	unsigned long flags;
 
-	i915_active_acquire(ref);
+	GEM_BUG_ON(i915_active_is_idle(ref));
 
-	llist_for_each_safe(pos, next, llist_del_all(&ref->barriers)) {
-		struct intel_engine_cs *engine;
-		struct active_node *node;
+	/*
+	 * Transfer the list of preallocated barriers into the
+	 * i915_active rbtree, but only as proto-nodes. They will be
+	 * populated by i915_request_add_active_barriers() to point to the
+	 * request that will eventually release them.
+	 */
+	llist_for_each_safe(pos, next, take_preallocated_barriers(ref)) {
+		struct active_node *node = barrier_from_ll(pos);
+		struct intel_engine_cs *engine = barrier_to_engine(node);
 		struct rb_node **p, *parent;
 
-		node = container_of((struct list_head *)pos,
-				    typeof(*node), base.link);
-
-		engine = (void *)rcu_access_pointer(node->base.request);
-		RCU_INIT_POINTER(node->base.request, ERR_PTR(-EAGAIN));
-
+		spin_lock_irqsave_nested(&ref->tree_lock, flags,
+					 SINGLE_DEPTH_NESTING);
 		parent = NULL;
 		p = &ref->tree.rb_node;
 		while (*p) {
+			struct active_node *it;
+
 			parent = *p;
-			if (rb_entry(parent,
-				     struct active_node,
-				     node)->timeline < node->timeline)
+
+			it = rb_entry(parent, struct active_node, node);
+			if (it->timeline < node->timeline)
 				p = &parent->rb_right;
 			else
 				p = &parent->rb_left;
 		}
 		rb_link_node(&node->node, parent, p);
 		rb_insert_color(&node->node, &ref->tree);
+		spin_unlock_irqrestore(&ref->tree_lock, flags);
 
-		llist_add((struct llist_node *)&node->base.link,
-			  &engine->barrier_tasks);
+		GEM_BUG_ON(!intel_engine_pm_is_awake(engine));
+		llist_add(barrier_to_ll(node), &engine->barrier_tasks);
 		intel_engine_pm_put(engine);
 	}
-	i915_active_release(ref);
 }
 
-void i915_request_add_barriers(struct i915_request *rq)
+static struct dma_fence **ll_to_fence_slot(struct llist_node *node)
+{
+	return __active_fence_slot(&barrier_from_ll(node)->base);
+}
+
+void i915_request_add_active_barriers(struct i915_request *rq)
 {
 	struct intel_engine_cs *engine = rq->engine;
 	struct llist_node *node, *next;
+	unsigned long flags;
 
-	llist_for_each_safe(node, next, llist_del_all(&engine->barrier_tasks))
-		list_add_tail((struct list_head *)node, &rq->active_list);
+	GEM_BUG_ON(!intel_context_is_barrier(rq->context));
+	GEM_BUG_ON(intel_engine_is_virtual(engine));
+	GEM_BUG_ON(i915_request_timeline(rq) != engine->kernel_context->timeline);
+
+	node = llist_del_all(&engine->barrier_tasks);
+	if (!node)
+		return;
+	/*
+	 * Attach the list of proto-fences to the in-flight request such
+	 * that the parent i915_active will be released when this request
+	 * is retired.
+	 */
+	spin_lock_irqsave(&rq->lock, flags);
+	llist_for_each_safe(node, next, node) {
+		/* serialise with reuse_idle_barrier */
+		smp_store_mb(*ll_to_fence_slot(node), &rq->fence);
+		list_add_tail((struct list_head *)node, &rq->fence.cb_list);
+	}
+	spin_unlock_irqrestore(&rq->lock, flags);
 }
 
-int i915_active_request_set(struct i915_active_request *active,
-			    struct i915_request *rq)
+/*
+ * __i915_active_fence_set: Update the last active fence along its timeline
+ * @active: the active tracker
+ * @fence: the new fence (under construction)
+ *
+ * Records the new @fence as the last active fence along its timeline in
+ * this active tracker, moving the tracking callbacks from the previous
+ * fence onto this one. Returns the previous fence (if not already completed),
+ * which the caller must ensure is executed before the new fence. To ensure
+ * that the order of fences within the timeline of the i915_active_fence is
+ * understood, it should be locked by the caller.
+ */
+struct dma_fence *
+__i915_active_fence_set(struct i915_active_fence *active,
+			struct dma_fence *fence)
 {
-	int err;
+	struct dma_fence *prev;
+	unsigned long flags;
 
-	/* Must maintain ordering wrt previous active requests */
-	err = i915_request_await_active_request(rq, active);
-	if (err)
-		return err;
+	if (fence == rcu_access_pointer(active->fence))
+		return fence;
 
-	__i915_active_request_set(active, rq);
-	return 0;
+	GEM_BUG_ON(test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags));
+
+	/*
+	 * Consider that we have two threads arriving (A and B), with
+	 * C already resident as the active->fence.
+	 *
+	 * A does the xchg first, and so it sees C or NULL depending
+	 * on the timing of the interrupt handler. If it is NULL, the
+	 * previous fence must have been signaled and we know that
+	 * we are first on the timeline. If it is still present,
+	 * we acquire the lock on that fence and serialise with the interrupt
+	 * handler, in the process removing it from any future interrupt
+	 * callback. A will then wait on C before executing (if present).
+	 *
+	 * As B is second, it sees A as the previous fence and so waits for
+	 * it to complete its transition and takes over the occupancy for
+	 * itself -- remembering that it needs to wait on A before executing.
+	 *
+	 * Note the strong ordering of the timeline also provides consistent
+	 * nesting rules for the fence->lock; the inner lock is always the
+	 * older lock.
+	 */
+	spin_lock_irqsave(fence->lock, flags);
+	prev = xchg(__active_fence_slot(active), fence);
+	if (prev) {
+		GEM_BUG_ON(prev == fence);
+		spin_lock_nested(prev->lock, SINGLE_DEPTH_NESTING);
+		__list_del_entry(&active->cb.node);
+		spin_unlock(prev->lock); /* serialise with prev->cb_list */
+	}
+	GEM_BUG_ON(rcu_access_pointer(active->fence) != fence);
+	list_add_tail(&active->cb.node, &fence->cb_list);
+	spin_unlock_irqrestore(fence->lock, flags);
+
+	return prev;
+}
+
+int i915_active_fence_set(struct i915_active_fence *active,
+			  struct i915_request *rq)
+{
+	struct dma_fence *fence;
+	int err = 0;
+
+	/* Must maintain timeline ordering wrt previous active requests */
+	rcu_read_lock();
+	fence = __i915_active_fence_set(active, &rq->fence);
+	if (fence) /* but the previous fence may not belong to that timeline! */
+		fence = dma_fence_get_rcu(fence);
+	rcu_read_unlock();
+	if (fence) {
+		err = i915_request_await_dma_fence(rq, fence);
+		dma_fence_put(fence);
+	}
+
+	return err;
 }
 
-void i915_active_retire_noop(struct i915_active_request *active,
-			     struct i915_request *request)
+void i915_active_noop(struct dma_fence *fence, struct dma_fence_cb *cb)
 {
-	/* Space left intentionally blank */
+	active_fence_cb(fence, cb);
 }
 
 #if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)