From 3a7a8257613141edb03833c5d277b14ee28697dc Mon Sep 17 00:00:00 2001 From: Juergen Gross Date: Mon, 26 Jan 2026 08:45:17 +0100 Subject: [PATCH] xen/xenbus: better handle backend crash MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Signed-off-by: Juergen Gross --- drivers/xen/xenbus/xenbus_client.c | 9 ++++++-- drivers/xen/xenbus/xenbus_probe.c | 34 ++++++++++++++++++++++++++++++ include/xen/xenbus.h | 1 + 3 files changed, 42 insertions(+), 2 deletions(-) diff --git a/drivers/xen/xenbus/xenbus_client.c b/drivers/xen/xenbus/xenbus_client.c index 2dc874fb5506..0d99a1f60df4 100644 --- a/drivers/xen/xenbus/xenbus_client.c +++ b/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", ¤t_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; diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c index 86fe6e779056..fcd59a2109c3 100644 --- a/drivers/xen/xenbus/xenbus_probe.c +++ b/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 +662,37 @@ 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->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; + device_unregister(&dev->dev); + put_device(&dev->dev); + dev = NULL; + } + } if (!dev) xenbus_probe_node(bus, type, root); else diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h index c94caf852aea..064cb6db5e9f 100644 --- a/include/xen/xenbus.h +++ b/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; -- 2.52.0