6.6-stable review patch. If anyone has any objections, please let me know.
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From: Kuniyuki Iwashima kuniyu@amazon.com
commit 77e5593aebba823bcbcf2c4b58b07efcd63933b8 upstream.
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 Acked-by: Paolo Abeni pabeni@redhat.com Link: https://lore.kernel.org/r/20240325202425.60930-11-kuniyu@amazon.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Lee Jones lee@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/unix/garbage.c | 48 +++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 47 insertions(+), 1 deletion(-)
--- a/net/unix/garbage.c +++ b/net/unix/garbage.c @@ -112,6 +112,19 @@ static struct unix_vertex *unix_edge_suc 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_
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 @@ out: 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 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 @@ prev_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 @@ prev_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
spin_lock(&unix_gc_lock);
+ if (!unix_graph_maybe_cyclic) + goto skip_gc; + unix_walk_scc();
/* First, select candidates for garbage collection. Only @@ -633,7 +679,7 @@ static void __unix_gc(struct work_struct
/* 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);