[PATCH 1/3] drm/i915/active: Serialize preallocation of idle barriers

[Janusz Krzysztofik]

When we collect barriers for preallocating them, we reuse either idle or
non-idle ones, whichever we find.  In case of non-idle barriers, we
depend on their successful deletion from their barrier tasks lists as an
indication of them not being claimed by other threads.  However, in case
of idle barriers, we neither perform any similar checks nor take any
preventive countermeasures, then we risk unexpected races with other
threads, also those trying to replace an idle barrier with a request.  We
may then end up adding a barrier to two independent preallocated lists,
then adding them twice back to the engine's barrier tasks lists, thus
effectively creating a loop of llist nodes.  Or we can corrupt an active
fence pointer and/or corrupt its callback list pointers, or they can hurt
us in a similar way.

Whenever an idle barrier is selected for preallocation, set it up
immediately as non-idle, i.e., mark it with our ERR_PTR(-EAGAIN) barrier
mark, so other threads are no longer able to claim it as not present on
barrier tasks list.  Also, increase active count of its composite tracker
as long as we still know it was idle.  Serialize that claim operation with
potential concurrent updates of active fence pointer, and allocate a new
node if the idle one we found occurs claimed by another competing thread.

While being at it, fortify now still racy check for preallocated_barriers
llist being still empty when we populate it with collected proto-barriers
(assuming we need that check).

Fixes: 9ff33bbcda25 ("drm/i915: Reduce locking around i915_active_acquire_preallocate_barrier()")
Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik at linux.intel.com>
Cc: stable at vger.kernel.org # v5.10
---
 drivers/gpu/drm/i915/i915_active.c | 50 +++++++++++++++++-------------
 1 file changed, 29 insertions(+), 21 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
index 7412abf166a8c..7e2fee428ff35 100644
--- a/drivers/gpu/drm/i915/i915_active.c
+++ b/drivers/gpu/drm/i915/i915_active.c
@@ -785,8 +785,13 @@ static struct active_node *reuse_idle_barrier(struct i915_active *ref, u64 idx)
 	 * 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.
+	 *
+	 * We reuse the request field to mark this as being our proto-node.
 	 */
-	if (ref->cache && is_idle_barrier(ref->cache, idx)) {
+	if (ref->cache && is_idle_barrier(ref->cache, idx) &&
+	    !cmpxchg(__active_fence_slot(&ref->cache->base), NULL,
+		     ERR_PTR(-EAGAIN))) {
+		__i915_active_acquire(ref);
 		p = &ref->cache->node;
 		goto match;
 	}
@@ -797,8 +802,12 @@ static struct active_node *reuse_idle_barrier(struct i915_active *ref, u64 idx)
 		struct active_node *node =
 			rb_entry(p, struct active_node, node);
 
-		if (is_idle_barrier(node, idx))
+		if (is_idle_barrier(node, idx) &&
+		    !cmpxchg(__active_fence_slot(&node->base), NULL,
+			     ERR_PTR(-EAGAIN))) {
+			__i915_active_acquire(ref);
 			goto match;
+		}
 
 		prev = p;
 		if (node->timeline < idx)
@@ -824,8 +833,12 @@ static struct active_node *reuse_idle_barrier(struct i915_active *ref, u64 idx)
 		if (node->timeline < idx)
 			continue;
 
-		if (is_idle_barrier(node, idx))
+		if (is_idle_barrier(node, idx) &&
+		    !cmpxchg(__active_fence_slot(&node->base), NULL,
+			     ERR_PTR(-EAGAIN))) {
+			__i915_active_acquire(ref);
 			goto match;
+		}
 
 		/*
 		 * The list of pending barriers is protected by the
@@ -886,29 +899,24 @@ int i915_active_acquire_preallocate_barrier(struct i915_active *ref,
 			if (!node)
 				goto unwind;
 
-			RCU_INIT_POINTER(node->base.fence, NULL);
+			/* Mark this as being our unconnected proto-node */
+			RCU_INIT_POINTER(node->base.fence, ERR_PTR(-EAGAIN));
 			node->base.cb.func = node_retire;
 			node->timeline = idx;
 			node->ref = ref;
-		}
-
-		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;
 			__i915_active_acquire(ref);
+		} else {
+			GEM_BUG_ON(rcu_access_pointer(node->base.fence) !=
+				   ERR_PTR(-EAGAIN));
 		}
-		GEM_BUG_ON(rcu_access_pointer(node->base.fence) != ERR_PTR(-EAGAIN));
 
-		GEM_BUG_ON(barrier_to_engine(node) != engine);
+		/*
+		 * Since this node is not in any list, we have decoupled it
+		 * from the rbtree, and we reuse the request to indicate
+		 * this is a barrier node, then we can use list pointers
+		 * for our tracking of the pending barrier.
+		 */
+		node->base.cb.node.prev = (void *)engine;
 		first = barrier_to_ll(node);
 		first->next = prev;
 		if (!last)
@@ -917,7 +925,7 @@ int i915_active_acquire_preallocate_barrier(struct i915_active *ref,
 	}
 
 	GEM_BUG_ON(!llist_empty(&ref->preallocated_barriers));
-	llist_add_batch(first, last, &ref->preallocated_barriers);
+	GEM_BUG_ON(!llist_add_batch(first, last, &ref->preallocated_barriers));
 
 	return 0;
 
-- 
2.25.1