xen: perform XenDevice clean-up in XenBus watch handler

Cleaning up offline XenDevice objects directly in
xen_device_backend_changed() is dangerous as xen_device_unrealize() will
modify the watch list that is being walked. Even the QLIST_FOREACH_SAFE()
used in notifier_list_notify() is insufficient as *two* notifiers (for
the frontend and backend watches) are removed, thus potentially rendering
the 'next' pointer unsafe.

The solution is to use the XenBus backend_watch handler to do the clean-up
instead, as it is invoked whilst walking a separate watch list.

This patch therefore adds a new 'inactive_devices' list to XenBus, to which
offline devices are added by xen_device_backend_changed(). The XenBus
backend_watch registration is also changed to not only invoke
xen_bus_enumerate() but also a new xen_bus_cleanup() function, which will
walk 'inactive_devices' and perform the necessary actions.
For safety an extra 'online' check is also added to xen_bus_type_enumerate()
to make sure that no attempt is made to create a new XenDevice object for a
backend that is offline.

NOTE: This patch also includes some cosmetic changes:
      - substitute the local variable name 'backend_state'
        in xen_bus_type_enumerate() with 'state', since there
        is no ambiguity with any other state in that context.
      - change xen_device_state_is_active() to
        xen_device_frontend_is_active() (and pass a XenDevice directly)
        since the state tests contained therein only apply to a frontend.
      - use 'state' rather then 'xendev->backend_state' in
        xen_device_backend_changed() to shorten the code.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20190913082159.31338-4-paul.durrant@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

3 files changed, 74 insertions, 25 deletions

diff --git a/hw/xen/trace-events b/hw/xen/trace-events
index 80ce3dafad..e6885bc751 100644
--- a/hw/xen/trace-events
+++ b/hw/xen/trace-events
@@ -17,8 +17,10 @@ xen_domid_restrict(int err) "err: %u"
 xen_bus_realize(void) ""
 xen_bus_unrealize(void) ""
 xen_bus_enumerate(void) ""
+xen_bus_cleanup(void) ""
 xen_bus_type_enumerate(const char *type) "type: %s"
 xen_bus_backend_create(const char *type, const char *path) "type: %s path: %s"
+xen_bus_device_cleanup(const char *type, char *name) "type: %s name: %s"
 xen_bus_add_watch(const char *node, const char *key) "node: %s key: %s"
 xen_bus_remove_watch(const char *node, const char *key) "node: %s key: %s"
 xen_device_realize(const char *type, char *name) "type: %s name: %s"
diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
index 810a4e2df3..55c157393d 100644
--- a/hw/xen/xen-bus.c
+++ b/hw/xen/xen-bus.c
@@ -340,13 +340,18 @@ static void xen_bus_type_enumerate(XenBus *xenbus, const char *type)
     for (i = 0; i < n; i++) {
         char *backend_path = g_strdup_printf("%s/%s", domain_path,
                                              backend[i]);
-        enum xenbus_state backend_state;
+        enum xenbus_state state;
+        unsigned int online;
 
         if (xs_node_scanf(xenbus->xsh, XBT_NULL, backend_path, "state",
-                          NULL, "%u", &backend_state) != 1)
-            backend_state = XenbusStateUnknown;
+                          NULL, "%u", &state) != 1)
+            state = XenbusStateUnknown;
 
-        if (backend_state == XenbusStateInitialising) {
+        if (xs_node_scanf(xenbus->xsh, XBT_NULL, backend_path, "online",
+                          NULL, "%u", &online) != 1)
+            online = 0;
+
+        if (online && state == XenbusStateInitialising) {
             Error *local_err = NULL;
 
             xen_bus_backend_create(xenbus, type, backend[i], backend_path,
@@ -365,9 +370,8 @@ out:
     g_free(domain_path);
 }
 
-static void xen_bus_enumerate(void *opaque)
+static void xen_bus_enumerate(XenBus *xenbus)
 {
-    XenBus *xenbus = opaque;
     char **type;
     unsigned int i, n;
 
@@ -385,6 +389,45 @@ static void xen_bus_enumerate(void *opaque)
     free(type);
 }
 
+static void xen_bus_device_cleanup(XenDevice *xendev)
+{
+    const char *type = object_get_typename(OBJECT(xendev));
+    Error *local_err = NULL;
+
+    trace_xen_bus_device_cleanup(type, xendev->name);
+
+    g_assert(!xendev->backend_online);
+
+    if (!xen_backend_try_device_destroy(xendev, &local_err)) {
+        object_unparent(OBJECT(xendev));
+    }
+
+    if (local_err) {
+        error_report_err(local_err);
+    }
+}
+
+static void xen_bus_cleanup(XenBus *xenbus)
+{
+    XenDevice *xendev, *next;
+
+    trace_xen_bus_cleanup();
+
+    QLIST_FOREACH_SAFE(xendev, &xenbus->inactive_devices, list, next) {
+        g_assert(xendev->inactive);
+        QLIST_REMOVE(xendev, list);
+        xen_bus_device_cleanup(xendev);
+    }
+}
+
+static void xen_bus_backend_changed(void *opaque)
+{
+    XenBus *xenbus = opaque;
+
+    xen_bus_enumerate(xenbus);
+    xen_bus_cleanup(xenbus);
+}
+
 static void xen_bus_unrealize(BusState *bus, Error **errp)
 {
     XenBus *xenbus = XEN_BUS(bus);
@@ -433,7 +476,7 @@ static void xen_bus_realize(BusState *bus, Error **errp)
 
     xenbus->backend_watch =
         xen_bus_add_watch(xenbus, "", /* domain root node */
-                          "backend", xen_bus_enumerate, &local_err);
+                          "backend", xen_bus_backend_changed, &local_err);
     if (local_err) {
         /* This need not be treated as a hard error so don't propagate */
         error_reportf_err(local_err,
@@ -555,9 +598,9 @@ static void xen_device_backend_set_online(XenDevice *xendev, bool online)
  * Tell from the state whether the frontend is likely alive,
  * i.e. it will react to a change of state of the backend.
  */
-static bool xen_device_state_is_active(enum xenbus_state state)
+static bool xen_device_frontend_is_active(XenDevice *xendev)
 {
-    switch (state) {
+    switch (xendev->frontend_state) {
     case XenbusStateInitWait:
     case XenbusStateInitialised:
     case XenbusStateConnected:
@@ -594,30 +637,31 @@ static void xen_device_backend_changed(void *opaque)
      * state to Closing, but there is no active frontend then set the
      * backend state to Closed.
      */
-    if (xendev->backend_state == XenbusStateClosing &&
-        !xen_device_state_is_active(xendev->frontend_state)) {
+    if (state == XenbusStateClosing &&
+        !xen_device_frontend_is_active(xendev)) {
         xen_device_backend_set_state(xendev, XenbusStateClosed);
     }
 
     /*
      * If a backend is still 'online' then we should leave it alone but,
-     * if a backend is not 'online', then the device should be destroyed
-     * once the state is Closed.
+     * if a backend is not 'online', then the device is a candidate
+     * for destruction. Hence add it to the 'inactive' list to be cleaned
+     * by xen_bus_cleanup().
      */
-    if (!xendev->backend_online &&
-        (xendev->backend_state == XenbusStateClosed ||
-         xendev->backend_state == XenbusStateInitialising ||
-         xendev->backend_state == XenbusStateInitWait ||
-         xendev->backend_state == XenbusStateUnknown)) {
-        Error *local_err = NULL;
+    if (!online &&
+        (state == XenbusStateClosed ||  state == XenbusStateInitialising ||
+         state == XenbusStateInitWait || state == XenbusStateUnknown) &&
+        !xendev->inactive) {
+        XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
 
-        if (!xen_backend_try_device_destroy(xendev, &local_err)) {
-            object_unparent(OBJECT(xendev));
-        }
+        xendev->inactive = true;
+        QLIST_INSERT_HEAD(&xenbus->inactive_devices, xendev, list);
 
-        if (local_err) {
-            error_report_err(local_err);
-        }
+        /*
+         * Re-write the state to cause a XenBus backend_watch notification,
+         * resulting in a call to xen_bus_cleanup().
+         */
+        xen_device_backend_printf(xendev, "state", "%u", state);
     }
 }
 
diff --git a/include/hw/xen/xen-bus.h b/include/hw/xen/xen-bus.h
index 0d198148f6..3d5532258d 100644
--- a/include/hw/xen/xen-bus.h
+++ b/include/hw/xen/xen-bus.h
@@ -32,7 +32,9 @@ typedef struct XenDevice {
     XenWatch *backend_online_watch;
     xengnttab_handle *xgth;
     bool feature_grant_copy;
+    bool inactive;
     QLIST_HEAD(, XenEventChannel) event_channels;
+    QLIST_ENTRY(XenDevice) list;
 } XenDevice;
 
 typedef char *(*XenDeviceGetName)(XenDevice *xendev, Error **errp);
@@ -68,6 +70,7 @@ typedef struct XenBus {
     struct xs_handle *xsh;
     XenWatchList *watch_list;
     XenWatch *backend_watch;
+    QLIST_HEAD(, XenDevice) inactive_devices;
 } XenBus;
 
 typedef struct XenBusClass {