netdev - [PATCH v2 2/2] Revert "xen-netback: Check for hotplug-status existence before watching"

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Message-Id: <20220222001817.2264967-2-marmarek@invisiblethingslab.com>
Date:   Tue, 22 Feb 2022 01:18:17 +0100
From:   Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
To:     linux-kernel@...r.kernel.org
Cc:     Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>, stable@...r.kernel.org,
        Michael Brown <mcb30@...e.org>, Wei Liu <wei.liu@...nel.org>,
        Paul Durrant <paul@....org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        xen-devel@...ts.xenproject.org (moderated list:XEN NETWORK BACKEND
        DRIVER),
        netdev@...r.kernel.org (open list:XEN NETWORK BACKEND DRIVER)
Subject: [PATCH v2 2/2] Revert "xen-netback: Check for hotplug-status existence before watching"

This reverts commit 2afeec08ab5c86ae21952151f726bfe184f6b23d.

The reasoning in the commit was wrong - the code expected to setup the
watch even if 'hotplug-status' didn't exist. In fact, it relied on the
watch being fired the first time - to check if maybe 'hotplug-status' is
already set to 'connected'. Not registering a watch for non-existing
path (which is the case if hotplug script hasn't been executed yet),
made the backend not waiting for the hotplug script to execute. This in
turns, made the netfront think the interface is fully operational, while
in fact it was not (the vif interface on xen-netback side might not be
configured yet).

This was a workaround for 'hotplug-status' erroneously being removed.
But since that is reverted now, the workaround is not necessary either.

More discussion at
https://lore.kernel.org/xen-devel/afedd7cb-a291-e773-8b0d-4db9b291fa98@ipxe.org/T/#u

Cc: stable@...r.kernel.org
Signed-off-by: Marek Marczykowski-Górecki <marmarek@...isiblethingslab.com>
---
I believe this is the same issue as discussed at
https://lore.kernel.org/xen-devel/20220113111946.GA4133739@dingwall.me.uk/
Cc: James Dingwall <james-xen@...gwall.me.uk
Cc: Michael Brown <mcb30@...e.org>
---
 drivers/net/xen-netback/xenbus.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/drivers/net/xen-netback/xenbus.c b/drivers/net/xen-netback/xenbus.c
index 3fad58d22155..990360d75cb6 100644
--- a/drivers/net/xen-netback/xenbus.c
+++ b/drivers/net/xen-netback/xenbus.c
@@ -824,15 +824,11 @@ static void connect(struct backend_info *be)
 	xenvif_carrier_on(be->vif);
 
 	unregister_hotplug_status_watch(be);
-	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;
+	err = xenbus_watch_pathfmt(dev, &be->hotplug_status_watch, NULL,
+				   hotplug_status_changed,
+				   "%s/%s", dev->nodename, "hotplug-status");
+	if (!err)
 		be->have_hotplug_status_watch = 1;
-	}
 
 	netif_tx_wake_all_queues(be->vif->dev);
 
-- 
2.31.1