net-sysfs: prevent uncleared queues from being re-added

With the (upcoming) removal of the rtnl_trylock/restart_syscall logic
and because of how Tx/Rx queues are implemented (and their
requirements), it might happen that a queue is re-added before having
the chance to be cleared. In such rare case, do not complete the queue
addition operation.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
Link: https://patch.msgid.link/20250204170314.146022-4-atenart@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 32 insertions, 0 deletions

diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index 0b7ee260613d..027af27517fa 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -1210,6 +1210,22 @@ static int rx_queue_add_kobject(struct net_device *dev, int index)
 	struct kobject *kobj = &queue->kobj;
 	int error = 0;
 
+	/* Rx queues are cleared in rx_queue_release to allow later
+	 * re-registration. This is triggered when their kobj refcount is
+	 * dropped.
+	 *
+	 * If a queue is removed while both a read (or write) operation and a
+	 * the re-addition of the same queue are pending (waiting on rntl_lock)
+	 * it might happen that the re-addition will execute before the read,
+	 * making the initial removal to never happen (queue's kobj refcount
+	 * won't drop enough because of the pending read). In such rare case,
+	 * return to allow the removal operation to complete.
+	 */
+	if (unlikely(kobj->state_initialized)) {
+		netdev_warn_once(dev, "Cannot re-add rx queues before their removal completed");
+		return -EAGAIN;
+	}
+
 	/* Kobject_put later will trigger rx_queue_release call which
 	 * decreases dev refcount: Take that reference here
 	 */
@@ -1898,6 +1914,22 @@ static int netdev_queue_add_kobject(struct net_device *dev, int index)
 	struct kobject *kobj = &queue->kobj;
 	int error = 0;
 
+	/* Tx queues are cleared in netdev_queue_release to allow later
+	 * re-registration. This is triggered when their kobj refcount is
+	 * dropped.
+	 *
+	 * If a queue is removed while both a read (or write) operation and a
+	 * the re-addition of the same queue are pending (waiting on rntl_lock)
+	 * it might happen that the re-addition will execute before the read,
+	 * making the initial removal to never happen (queue's kobj refcount
+	 * won't drop enough because of the pending read). In such rare case,
+	 * return to allow the removal operation to complete.
+	 */
+	if (unlikely(kobj->state_initialized)) {
+		netdev_warn_once(dev, "Cannot re-add tx queues before their removal completed");
+		return -EAGAIN;
+	}
+
 	/* Kobject_put later will trigger netdev_queue_release call
 	 * which decreases dev refcount: Take that reference here
 	 */