[PATCH 6.18 17/83] af_unix: Simplify GC state.

Greg Kroah-Hartman
2026-04-13T16:07:32Z

From: Kuniyuki Iwashima <kuniyu@google.com>

[ Upstream commit 6b6f3c71fe568aa8ed3e16e9135d88a5f4fd3e84 ]

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>
Link: https://patch.msgid.link/20251115020935.2643121-3-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: e5b31d988a41 ("af_unix: Give up GC if MSG_PEEK intervened.")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 9f62d50979735..7528e2db1293f 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);
--
2.53.0