[RFC PATCH net-next 3/4] net-sysfs: prevent uncleared queues from being re-added

[Antoine Tenart <atenart@kernel.org>]

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>
---
 net/core/net-sysfs.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index a9f712ef9925..75fb92c44291 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -1160,6 +1160,20 @@ 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))
+		return -EAGAIN;
+
 	/* Kobject_put later will trigger rx_queue_release call which
 	 * decreases dev refcount: Take that reference here
 	 */
@@ -1775,6 +1789,20 @@ 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))
+		return -EAGAIN;
+
 	/* Kobject_put later will trigger netdev_queue_release call
 	 * which decreases dev refcount: Take that reference here
 	 */
-- 
2.41.0