[PATCH v1 net-next 2/7] af_unix: Simplify GC state.

[Kuniyuki Iwashima <kuniyu@google.com>]

GC manages its state by two variables, unix_graph_maybe_cyclic
and unix_graph_grouped, both of which are set to false in the
initial state.

When an AF_UNIX socket is passed to an in-flight AF_UNIX socket,
unix_update_graph() sets unix_graph_maybe_cyclic to true and
unix_graph_grouped to false, making the next GC invocation call
unix_walk_scc() to group SCCs.

Once unix_walk_scc() finishes, sockets in the same SCC are linked
via vertex->scc_entry.  Then, unix_graph_grouped is set to true
so that the following GC invocations can skip Tarjan's algorithm
and simply iterate through the list in unix_walk_scc_fast().

In addition, if we know there is at least one cyclic reference,
we set unix_graph_maybe_cyclic to true so that we do not skip GC.

So the state transitions as follows:

  (unix_graph_maybe_cyclic, unix_graph_grouped)
  =
  (false, false) -> (true, false) -> (true, true) or (false, true)
                         ^.______________/________________/

There is no transition to the initial state where both variables
are false.

If we consider the initial state as grouped, we can see that the
GC actually has a tristate.

Let's consolidate two variables into one enum.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/unix/garbage.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index 9f62d5097973..7528e2db1293 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -121,8 +121,13 @@ static struct unix_vertex *unix_edge_successor(struct unix_edge *edge)
 	return edge->successor->vertex;
 }
 
-static bool unix_graph_maybe_cyclic;
-static bool unix_graph_grouped;
+enum {
+	UNIX_GRAPH_NOT_CYCLIC,
+	UNIX_GRAPH_MAYBE_CYCLIC,
+	UNIX_GRAPH_CYCLIC,
+};
+
+static unsigned char unix_graph_state;
 
 static void unix_update_graph(struct unix_vertex *vertex)
 {
@@ -132,8 +137,7 @@ static void unix_update_graph(struct unix_vertex *vertex)
 	if (!vertex)
 		return;
 
-	unix_graph_maybe_cyclic = true;
-	unix_graph_grouped = false;
+	unix_graph_state = UNIX_GRAPH_MAYBE_CYCLIC;
 }
 
 static LIST_HEAD(unix_unvisited_vertices);
@@ -536,8 +540,7 @@ static void unix_walk_scc(struct sk_buff_head *hitlist)
 	swap(unix_vertex_unvisited_index, unix_vertex_grouped_index);
 
 	unix_graph_cyclic_sccs = cyclic_sccs;
-	unix_graph_maybe_cyclic = !!unix_graph_cyclic_sccs;
-	unix_graph_grouped = true;
+	unix_graph_state = cyclic_sccs ? UNIX_GRAPH_CYCLIC : UNIX_GRAPH_NOT_CYCLIC;
 }
 
 static void unix_walk_scc_fast(struct sk_buff_head *hitlist)
@@ -570,7 +573,7 @@ static void unix_walk_scc_fast(struct sk_buff_head *hitlist)
 	list_replace_init(&unix_visited_vertices, &unix_unvisited_vertices);
 
 	unix_graph_cyclic_sccs = cyclic_sccs;
-	unix_graph_maybe_cyclic = !!unix_graph_cyclic_sccs;
+	unix_graph_state = cyclic_sccs ? UNIX_GRAPH_CYCLIC : UNIX_GRAPH_NOT_CYCLIC;
 }
 
 static bool gc_in_progress;
@@ -582,14 +585,14 @@ static void __unix_gc(struct work_struct *work)
 
 	spin_lock(&unix_gc_lock);
 
-	if (!unix_graph_maybe_cyclic) {
+	if (unix_graph_state == UNIX_GRAPH_NOT_CYCLIC) {
 		spin_unlock(&unix_gc_lock);
 		goto skip_gc;
 	}
 
 	__skb_queue_head_init(&hitlist);
 
-	if (unix_graph_grouped)
+	if (unix_graph_state == UNIX_GRAPH_CYCLIC)
 		unix_walk_scc_fast(&hitlist);
 	else
 		unix_walk_scc(&hitlist);
-- 
2.52.0.rc1.455.g30608eb744-goog