@@ -112,6 +112,19 @@ static struct unix_vertex *unix_edge_successor(struct unix_edge *edge)
return edge->successor->vertex;
}
+static bool unix_graph_maybe_cyclic;
+
+static void unix_update_graph(struct unix_vertex *vertex)
+{
+ /* If the receiver socket is not inflight, no cyclic
+ * reference could be formed.
+ */
+ if (!vertex)
+ return;
+
+ unix_graph_maybe_cyclic = true;
+}
+
static LIST_HEAD(unix_unvisited_vertices);
enum unix_vertex_index {
@@ -138,12 +151,16 @@ static void unix_add_edge(struct scm_fp_list *fpl, struct unix_edge *edge)
vertex->out_degree++;
list_add_tail(&edge->vertex_entry, &vertex->edges);
+
+ unix_update_graph(unix_edge_successor(edge));
}
static void unix_del_edge(struct scm_fp_list *fpl, struct unix_edge *edge)
{
struct unix_vertex *vertex = edge->predecessor->vertex;
+ unix_update_graph(unix_edge_successor(edge));
+
list_del(&edge->vertex_entry);
vertex->out_degree--;
@@ -227,6 +244,7 @@ void unix_del_edges(struct scm_fp_list *fpl)
void unix_update_edges(struct unix_sock *receiver)
{
spin_lock(&unix_gc_lock);
+ unix_update_graph(unix_sk(receiver->listener)->vertex);
receiver->listener = NULL;
spin_unlock(&unix_gc_lock);
}
@@ -268,6 +286,26 @@ void unix_destroy_fpl(struct scm_fp_list *fpl)
unix_free_vertices(fpl);
}
+static bool unix_scc_cyclic(struct list_head *scc)
+{
+ struct unix_vertex *vertex;
+ struct unix_edge *edge;
+
+ /* SCC containing multiple vertices ? */
+ if (!list_is_singular(scc))
+ return true;
+
+ vertex = list_first_entry(scc, typeof(*vertex), scc_entry);
+
+ /* Self-reference or a embryo-listener circle ? */
+ list_for_each_entry(edge, &vertex->edges, vertex_entry) {
+ if (unix_edge_successor(edge) == vertex)
+ return true;
+ }
+
+ return false;
+}
+
static LIST_HEAD(unix_visited_vertices);
static unsigned long unix_vertex_grouped_index = UNIX_VERTEX_INDEX_MARK2;
@@ -353,6 +391,9 @@ static void __unix_walk_scc(struct unix_vertex *vertex)
vertex->index = unix_vertex_grouped_index;
}
+ if (!unix_graph_maybe_cyclic)
+ unix_graph_maybe_cyclic = unix_scc_cyclic(&scc);
+
list_del(&scc);
}
@@ -363,6 +404,8 @@ static void __unix_walk_scc(struct unix_vertex *vertex)
static void unix_walk_scc(void)
{
+ unix_graph_maybe_cyclic = false;
+
/* Visit every vertex exactly once.
* __unix_walk_scc() moves visited vertices to unix_visited_vertices.
*/
@@ -524,6 +567,9 @@ static void __unix_gc(struct work_struct *work)
spin_lock(&unix_gc_lock);
+ if (!unix_graph_maybe_cyclic)
+ goto skip_gc;
+
unix_walk_scc();
/* First, select candidates for garbage collection. Only
@@ -617,7 +663,7 @@ static void __unix_gc(struct work_struct *work)
/* All candidates should have been detached by now. */
WARN_ON_ONCE(!list_empty(&gc_candidates));
-
+skip_gc:
/* Paired with READ_ONCE() in wait_for_unix_gc(). */
WRITE_ONCE(gc_in_progress, false);
We do not need to run GC if there is no possible cyclic reference. We use unix_graph_maybe_cyclic to decide if we should run GC. If a fd of an AF_UNIX socket is passed to an already inflight AF_UNIX socket, they could form a cyclic reference. Then, we set true to unix_graph_maybe_cyclic and later run Tarjan's algorithm to group them into SCC. Once we run Tarjan's algorithm, we are 100% sure whether cyclic references exist or not. If there is no cycle, we set false to unix_graph_maybe_cyclic and can skip the entire garbage collection next time. When finalising SCC, we set true to unix_graph_maybe_cyclic if SCC consists of multiple vertices. Even if SCC is a single vertex, a cycle might exist as self-fd passing. Given the corner case is rare, we detect it by checking all edges of the vertex and set true to unix_graph_maybe_cyclic. With this change, __unix_gc() is just a spin_lock() dance in the normal usage. Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com> --- net/unix/garbage.c | 48 +++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 47 insertions(+), 1 deletion(-)