af_unix: Avoid Tarjan's algorithm if unnecessary.

Once a cyclic reference is formed, we need to run GC to check if
there is dead SCC.

However, we do not need to run Tarjan's algorithm if we know that
the shape of the inflight graph has not been changed.

If an edge is added/updated/deleted and the edge's successor is
inflight, we set false to unix_graph_grouped, which means we need
to re-classify SCC.

Once we finalise SCC, we set true to unix_graph_grouped.

While unix_graph_grouped is true, we can iterate the grouped
SCC using vertex->scc_entry in unix_walk_scc_fast().

list_add() and list_for_each_entry_reverse() uses seem weird, but
they are to keep the vertex order consistent and make writing test
easier.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20240325202425.60930-12-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/unix/garbage.c

@@ -113,6 +113,7 @@ static struct unix_vertex *unix_edge_successor(struct unix_edge *edge)
 }
 
 static bool unix_graph_maybe_cyclic;
+static bool unix_graph_grouped;
 
 static void unix_update_graph(struct unix_vertex *vertex)
 {
@@ -123,6 +124,7 @@ static void unix_update_graph(struct unix_vertex *vertex)
 		return;
 
 	unix_graph_maybe_cyclic = true;
+	unix_graph_grouped = false;
 }
 
 static LIST_HEAD(unix_unvisited_vertices);
@@ -144,6 +146,7 @@ static void unix_add_edge(struct scm_fp_list *fpl, struct unix_edge *edge)
 		vertex->index = unix_vertex_unvisited_index;
 		vertex->out_degree = 0;
 		INIT_LIST_HEAD(&vertex->edges);
+		INIT_LIST_HEAD(&vertex->scc_entry);
 
 		list_move_tail(&vertex->entry, &unix_unvisited_vertices);
 		edge->predecessor->vertex = vertex;
@@ -418,6 +421,26 @@ static void unix_walk_scc(void)
 
 	list_replace_init(&unix_visited_vertices, &unix_unvisited_vertices);
 	swap(unix_vertex_unvisited_index, unix_vertex_grouped_index);
+
+	unix_graph_grouped = true;
+}
+
+static void unix_walk_scc_fast(void)
+{
+	while (!list_empty(&unix_unvisited_vertices)) {
+		struct unix_vertex *vertex;
+		struct list_head scc;
+
+		vertex = list_first_entry(&unix_unvisited_vertices, typeof(*vertex), entry);
+		list_add(&scc, &vertex->scc_entry);
+
+		list_for_each_entry_reverse(vertex, &scc, scc_entry)
+			list_move_tail(&vertex->entry, &unix_visited_vertices);
+
+		list_del(&scc);
+	}
+
+	list_replace_init(&unix_visited_vertices, &unix_unvisited_vertices);
 }
 
 static LIST_HEAD(gc_candidates);
@@ -570,7 +593,10 @@ static void __unix_gc(struct work_struct *work)
 	if (!unix_graph_maybe_cyclic)
 		goto skip_gc;
 
-	unix_walk_scc();
+	if (unix_graph_grouped)
+		unix_walk_scc_fast();
+	else
+		unix_walk_scc();
 
 	/* First, select candidates for garbage collection.  Only
 	 * in-flight sockets are considered, and from those only ones