Re: [PATCH] reset PV devices in crash kernel

[Olaf Hering <olaf@aepfle.de>]

After triggering a crash dump in a HVM guest, the PV backend drivers
will remain in connected state.  When the kdump kernel starts the PV
drivers will skip such devices.  As a result, no root device is found
and the vmcore cant be saved.

With this change all frontend devices with state XenbusStateConnected
will be reset by changing the state file to Closing/Closed/Initializing.
This will trigger a disconnect in the backend drivers. Now the frontend
drivers will find the backend drivers in state Initwait and can connect.

Signed-off-by: Olaf Hering <olaf@aepfle.de>

---
v2:
 add missing newlines to new printk() output

 drivers/xen/xenbus/xenbus_comms.c |    4 +
 drivers/xen/xenbus/xenbus_probe.c |   96 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 99 insertions(+), 1 deletion(-)

Index: linux-2.6.18-xen.hg/drivers/xen/xenbus/xenbus_comms.c
===================================================================
--- linux-2.6.18-xen.hg.orig/drivers/xen/xenbus/xenbus_comms.c
+++ linux-2.6.18-xen.hg/drivers/xen/xenbus/xenbus_comms.c
@@ -234,7 +234,9 @@ int xb_init_comms(void)
 		printk(KERN_WARNING "XENBUS response ring is not quiescent "
 		       "(%08x:%08x): fixing up\n",
 		       intf->rsp_cons, intf->rsp_prod);
-		intf->rsp_cons = intf->rsp_prod;
+		/* breaks kdump */
+		if (!reset_devices)
+			intf->rsp_cons = intf->rsp_prod;
 	}
 
 	if (xenbus_irq)
Index: linux-2.6.18-xen.hg/drivers/xen/xenbus/xenbus_probe.c
===================================================================
--- linux-2.6.18-xen.hg.orig/drivers/xen/xenbus/xenbus_probe.c
+++ linux-2.6.18-xen.hg/drivers/xen/xenbus/xenbus_probe.c
@@ -854,11 +854,107 @@ void unregister_xenstore_notifier(struct
 }
 EXPORT_SYMBOL_GPL(unregister_xenstore_notifier);
 
+#ifdef CONFIG_CRASH_DUMP
+static DECLARE_WAIT_QUEUE_HEAD(be_state_wq);
+static int be_state;
+
+static void xenbus_reset_state_changed(struct xenbus_watch *w, const char **v, unsigned int l)
+{
+	xenbus_scanf(XBT_NIL, v[XS_WATCH_PATH], "", "%i", &be_state);
+	printk(KERN_INFO "XENBUS: %s %s\n", v[XS_WATCH_PATH], xenbus_strstate(be_state));
+	wake_up(&be_state_wq);
+}
+
+static int xenbus_reset_check_final(int *st)
+{
+	return *st == XenbusStateInitialising || *st == XenbusStateInitWait;
+}
+
+static void xenbus_reset_frontend_state(char *backend, char *frontend)
+{
+	struct xenbus_watch watch;
+
+	memset(&watch, 0, sizeof(watch));
+	watch.node = kasprintf(GFP_NOIO | __GFP_HIGH, "%s/state", backend);
+	if (!watch.node)
+		return;
+
+	watch.callback = xenbus_reset_state_changed;
+	be_state = XenbusStateUnknown;
+
+	printk(KERN_INFO "triggering reconnect on %s\n", backend);
+	register_xenbus_watch(&watch);
+
+	xenbus_printf(XBT_NIL, frontend, "state", "%d", XenbusStateClosing);
+	wait_event_interruptible(be_state_wq, be_state == XenbusStateClosing);
+
+	xenbus_printf(XBT_NIL, frontend, "state", "%d", XenbusStateClosed);
+	wait_event_interruptible(be_state_wq, be_state == XenbusStateClosed);
+
+	xenbus_printf(XBT_NIL, frontend, "state", "%d", XenbusStateInitialising);
+	wait_event_interruptible(be_state_wq, xenbus_reset_check_final(&be_state));
+
+	unregister_xenbus_watch(&watch);
+	printk(KERN_INFO "reconnect done on %s\n", backend);
+	kfree(watch.node);
+}
+
+static void xenbus_reset_check_state(char *frontend)
+{
+	int state, err;
+	char *backend;
+
+	err = xenbus_scanf(XBT_NIL, frontend, "state", "%i", &state);
+	/* frontend connected? */
+	if (err == 1 && state == XenbusStateConnected) {
+		backend = xenbus_read(XBT_NIL, frontend, "backend", NULL);
+		if (!backend || IS_ERR(backend))
+			return;
+		err = xenbus_scanf(XBT_NIL, backend, "state", "%i", &state);
+		/* backend connected? */
+		if (err == 1 && state == XenbusStateConnected)
+			xenbus_reset_frontend_state(backend, frontend);
+		kfree(backend);
+	}
+}
+
+static void xenbus_reset_state(void)
+{
+	char *frontend;
+	char **devclass, **dev;
+	int devclass_n, dev_n;
+	int i, j;
+
+	devclass = xenbus_directory(XBT_NIL, "device", "", &devclass_n);
+	if (IS_ERR(devclass))
+		return;
+
+	for (i = 0; i < devclass_n; i++) {
+		dev = xenbus_directory(XBT_NIL, "device", devclass[i], &dev_n);
+		if (IS_ERR(dev))
+			continue;
+		for (j = 0; j < dev_n; j++) {
+			frontend = kasprintf(GFP_NOIO|__GFP_HIGH, "device/%s/%s", devclass[i], dev[j]);
+			if (!frontend)
+				continue;
+			xenbus_reset_check_state(frontend);
+			kfree(frontend);
+		}
+		kfree(dev);
+	}
+	kfree(devclass);
+}
+#endif
 
 void xenbus_probe(void *unused)
 {
 	BUG_ON(!is_xenstored_ready());
 
+#ifdef CONFIG_CRASH_DUMP
+	/* reset devices in XenbusStateConnected state */
+	if (!is_initial_xendomain() && reset_devices)
+		xenbus_reset_state();
+#endif
 	/* Enumerate devices in xenstore and watch for changes. */
 	xenbus_probe_devices(&xenbus_frontend);
 	register_xenbus_watch(&fe_watch);