Re: [syzbot] [net?] WARNING in __unix_gc

[Hillf Danton <hdanton@sina.com>]

On Fri, 02 Feb 2024 05:26:28 -0800
> HEAD commit:    1701940b1a02 Merge branch 'tools-net-ynl-add-features-for-..
> git tree:       net-next
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12d6927be80000

#syz test 

--- x/net/unix/garbage.c
+++ y/net/unix/garbage.c
@@ -237,6 +237,8 @@ static void inc_inflight(struct unix_soc
 	usk->inflight++;
 }
 
+static int recycle;
+
 static void inc_inflight_move_tail(struct unix_sock *u)
 {
 	u->inflight++;
@@ -245,8 +247,10 @@ static void inc_inflight_move_tail(struc
 	 * of the list, so that it's checked even if it was already
 	 * passed over
 	 */
-	if (test_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags))
+	if (test_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags)) {
 		list_move_tail(&u->link, &gc_candidates);
+		recycle++;
+	}
 }
 
 static bool gc_in_progress;
@@ -259,6 +263,7 @@ static void __unix_gc(struct work_struct
 	struct list_head cursor;
 
 	spin_lock(&unix_gc_lock);
+	recycle = 0;
 
 	/* First, select candidates for garbage collection.  Only
 	 * in-flight sockets are considered, and from those only ones
@@ -342,7 +347,7 @@ static void __unix_gc(struct work_struct
 	spin_lock(&unix_gc_lock);
 
 	/* All candidates should have been detached by now. */
-	WARN_ON_ONCE(!list_empty(&gc_candidates));
+	WARN_ON_ONCE(!list_empty(&gc_candidates) && !recycle);
 
 	/* Paired with READ_ONCE() in wait_for_unix_gc(). */
 	WRITE_ONCE(gc_in_progress, false);
--