netdev - [PATCH] xen-netback: Check for hotplug-status existence before watching

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Message-Id: <20210413152512.903750-1-mbrown@fensystems.co.uk>
Date:   Tue, 13 Apr 2021 16:25:12 +0100
From:   Michael Brown <mbrown@...systems.co.uk>
To:     paul@....org, xen-devel@...ts.xenproject.org,
        netdev@...r.kernel.org, wei.liu@...nel.org, pdurrant@...zon.com
Cc:     Michael Brown <mbrown@...systems.co.uk>
Subject: [PATCH] xen-netback: Check for hotplug-status existence before watching

The logic in connect() is currently written with the assumption that
xenbus_watch_pathfmt() will return an error for a node that does not
exist.  This assumption is incorrect: xenstore does allow a watch to
be registered for a nonexistent node (and will send notifications
should the node be subsequently created).

As of commit 1f2565780 ("xen-netback: remove 'hotplug-status' once it
has served its purpose"), this leads to a failure when a domU
transitions into XenbusStateConnected more than once.  On the first
domU transition into Connected state, the "hotplug-status" node will
be deleted by the hotplug_status_changed() callback in dom0.  On the
second or subsequent domU transition into Connected state, the
hotplug_status_changed() callback will therefore never be invoked, and
so the backend will remain stuck in InitWait.

This failure prevents scenarios such as reloading the xen-netfront
module within a domU, or booting a domU via iPXE.  There is
unfortunately no way for the domU to work around this dom0 bug.

Fix by explicitly checking for existence of the "hotplug-status" node,
thereby creating the behaviour that was previously assumed to exist.

Signed-off-by: Michael Brown <mbrown@...systems.co.uk>
---
 drivers/net/xen-netback/xenbus.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/net/xen-netback/xenbus.c b/drivers/net/xen-netback/xenbus.c
index a5439c130130..d24b7a7993aa 100644
--- a/drivers/net/xen-netback/xenbus.c
+++ b/drivers/net/xen-netback/xenbus.c
@@ -824,11 +824,15 @@ static void connect(struct backend_info *be)
 	xenvif_carrier_on(be->vif);

 	unregister_hotplug_status_watch(be);
-	err = xenbus_watch_pathfmt(dev, &be->hotplug_status_watch, NULL,
-				   hotplug_status_changed,
-				   "%s/%s", dev->nodename, "hotplug-status");
-	if (!err)
+	if (xenbus_exists(XBT_NIL, dev->nodename, "hotplug-status")) {
+		err = xenbus_watch_pathfmt(dev, &be->hotplug_status_watch,
+					   NULL, hotplug_status_changed,
+					   "%s/%s", dev->nodename,
+					   "hotplug-status");
+		if (err)
+			goto err;
 		be->have_hotplug_status_watch = 1;
+	}

 	netif_tx_wake_all_queues(be->vif->dev);

-- 
2.29.2