xen/xenbus: better handle backend crash

When the backend domain crashes, coordinated device cleanup is not
possible (as it involves waiting for the backend state change). In that
case, toolstack forcefully removes frontend xenstore entries.
xenbus_dev_changed() handles this case, and triggers device cleanup.
It's possible that toolstack manages to connect new device in that
place, before xenbus_dev_changed() notices the old one is missing. If
that happens, new one won't be probed and will forever remain in
XenbusStateInitialising.

Fix this by checking the frontend's state in Xenstore. In case it has
been reset to XenbusStateInitialising by Xen tools, consider this
being the result of an unplug+plug operation.

It's important that cleanup on such unplug doesn't modify Xenstore
entries (especially the "state" key) as it belong to the new device
to be probed - changing it would derail establishing connection to the
new backend (most likely, closing the device before it was even
connected). Handle this case by setting new xenbus_device->vanished
flag to true, and check it before changing state entry.

And even if xenbus_dev_changed() correctly detects the device was
forcefully removed, the cleanup handling is still racy. Since this whole
handling doesn't happened in a single Xenstore transaction, it's possible
that toolstack might put a new device there already. Avoid re-creating
the state key (which in the case of loosing the race would actually
close newly attached device).

The problem does not apply to frontend domain crash, as this case
involves coordinated cleanup.

Problem originally reported at
https://lore.kernel.org/xen-devel/aOZvivyZ9YhVWDLN@mail-itl/T/#t,
including reproduction steps.

Based-on-patch-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20260218095205.453657-3-jgross@suse.com>

Author: jgross1

drivers/xen/xenbus/xenbus_client.c

@@ -226,8 +226,9 @@ __xenbus_switch_state(struct xenbus_device *dev,
 	struct xenbus_transaction xbt;
 	int current_state;
 	int err, abort;
+	bool vanished = false;
 
-	if (state == dev->state)
+	if (state == dev->state || dev->vanished)
 		return 0;
 
 again:
@@ -242,6 +243,10 @@ __xenbus_switch_state(struct xenbus_device *dev,
 	err = xenbus_scanf(xbt, dev->nodename, "state", "%d", &current_state);
 	if (err != 1)
 		goto abort;
+	if (current_state != dev->state && current_state == XenbusStateInitialising) {
+		vanished = true;
+		goto abort;
+	}
 
 	err = xenbus_printf(xbt, dev->nodename, "state", "%d", state);
 	if (err) {
@@ -256,7 +261,7 @@ __xenbus_switch_state(struct xenbus_device *dev,
 		if (err == -EAGAIN && !abort)
 			goto again;
 		xenbus_switch_fatal(dev, depth, err, "ending transaction");
-	} else
+	} else if (!vanished)
 		dev->state = state;
 
 	return 0;
@@ -941,6 +946,10 @@ enum xenbus_state xenbus_read_driver_state(const struct xenbus_device *dev,
 					   const char *path)
 {
 	enum xenbus_state result;
+
+	if (dev && dev->vanished)
+		return XenbusStateUnknown;
+
 	int err = xenbus_gather(XBT_NIL, path, "state", "%d", &result, NULL);
 	if (err)
 		result = XenbusStateUnknown;

drivers/xen/xenbus/xenbus_probe.c

@@ -444,6 +444,9 @@ static void xenbus_cleanup_devices(const char *path, struct bus_type *bus)
 		info.dev = NULL;
 		bus_for_each_dev(bus, NULL, &info, cleanup_dev);
 		if (info.dev) {
+			dev_warn(&info.dev->dev,
+				 "device forcefully removed from xenstore\n");
+			info.dev->vanished = true;
 			device_unregister(&info.dev->dev);
 			put_device(&info.dev->dev);
 		}
@@ -659,6 +662,39 @@ void xenbus_dev_changed(const char *node, struct xen_bus_type *bus)
 		return;
 
 	dev = xenbus_device_find(root, &bus->bus);
+	/*
+	 * Backend domain crash results in not coordinated frontend removal,
+	 * without going through XenbusStateClosing. If this is a new instance
+	 * of the same device Xen tools will have reset the state to
+	 * XenbusStateInitializing.
+	 * It might be that the backend crashed early during the init phase of
+	 * device setup, in which case the known state would have been
+	 * XenbusStateInitializing. So test the backend domid to match the
+	 * saved one. In case the new backend happens to have the same domid as
+	 * the old one, we can just carry on, as there is no inconsistency
+	 * resulting in this case.
+	 */
+	if (dev && !strcmp(bus->root, "device")) {
+		enum xenbus_state state = xenbus_read_driver_state(dev, dev->nodename);
+		unsigned int backend = xenbus_read_unsigned(root, "backend-id",
+							    dev->otherend_id);
+
+		if (state == XenbusStateInitialising &&
+		    (state != dev->state || backend != dev->otherend_id)) {
+			/*
+			 * State has been reset, assume the old one vanished
+			 * and new one needs to be probed.
+			 */
+			dev_warn(&dev->dev,
+				 "state reset occurred, reconnecting\n");
+			dev->vanished = true;
+		}
+		if (dev->vanished) {
+			device_unregister(&dev->dev);
+			put_device(&dev->dev);
+			dev = NULL;
+		}
+	}
 	if (!dev)
 		xenbus_probe_node(bus, type, root);
 	else

include/xen/xenbus.h

@@ -80,6 +80,7 @@ struct xenbus_device {
 	const char *devicetype;
 	const char *nodename;
 	const char *otherend;
+	bool vanished;
 	int otherend_id;
 	struct xenbus_watch otherend_watch;
 	struct device dev;