xen / notify: introduce a new XenWatchList abstraction

Xenstore watch call-backs are already abstracted away from XenBus using
the XenWatch data structure but the associated NotifierList manipulation
and file handle registration is still open coded in various xen_bus_...()
functions.
This patch creates a new XenWatchList data structure to allow these
interactions to be abstracted away from XenBus as well. This is in
preparation for a subsequent patch which will introduce separate watch lists
for XenBus and XenDevice objects.

NOTE: This patch also introduces a new notifier_list_empty() helper function
      for the purposes of adding an assertion that a XenWatchList is not
      freed whilst its associated NotifierList is still occupied.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Anthony Perard <anthony.perard@citrix.com>
Message-Id: <20190913082159.31338-2-paul.durrant@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

hw/xen/trace-events

@@ -19,9 +19,8 @@ xen_bus_unrealize(void) ""
 xen_bus_enumerate(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_add_watch(const char *node, const char *key, char *token) "node: %s key: %s token: %s"
-xen_bus_remove_watch(const char *node, const char *key, char *token) "node: %s key: %s token: %s"
-xen_bus_watch(const char *token) "token: %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"
 xen_device_unrealize(const char *type, char *name) "type: %s name: %s"
 xen_device_backend_state(const char *type, char *name, const char *state) "type: %s name: %s -> %s"

hw/xen/xen-bus.c

@@ -157,18 +157,60 @@ static void free_watch(XenWatch *watch)
     g_free(watch);
 }
 
-static XenWatch *xen_bus_add_watch(XenBus *xenbus, const char *node,
+struct XenWatchList {
+    struct xs_handle *xsh;
+    NotifierList notifiers;
+};
+
+static void watch_list_event(void *opaque)
+{
+    XenWatchList *watch_list = opaque;
+    char **v;
+    const char *token;
+
+    v = xs_check_watch(watch_list->xsh);
+    if (!v) {
+        return;
+    }
+
+    token = v[XS_WATCH_TOKEN];
+
+    notifier_list_notify(&watch_list->notifiers, (void *)token);
+
+    free(v);
+}
+
+static XenWatchList *watch_list_create(struct xs_handle *xsh)
+{
+    XenWatchList *watch_list = g_new0(XenWatchList, 1);
+
+    g_assert(xsh);
+
+    watch_list->xsh = xsh;
+    notifier_list_init(&watch_list->notifiers);
+    qemu_set_fd_handler(xs_fileno(watch_list->xsh), watch_list_event, NULL,
+                        watch_list);
+
+    return watch_list;
+}
+
+static void watch_list_destroy(XenWatchList *watch_list)
+{
+    g_assert(notifier_list_empty(&watch_list->notifiers));
+    qemu_set_fd_handler(xs_fileno(watch_list->xsh), NULL, NULL, NULL);
+    g_free(watch_list);
+}
+
+static XenWatch *watch_list_add(XenWatchList *watch_list, const char *node,
                                 const char *key, XenWatchHandler handler,
                                 void *opaque, Error **errp)
 {
     XenWatch *watch = new_watch(node, key, handler, opaque);
     Error *local_err = NULL;
 
-    trace_xen_bus_add_watch(watch->node, watch->key, watch->token);
+    notifier_list_add(&watch_list->notifiers, &watch->notifier);
 
-    notifier_list_add(&xenbus->watch_notifiers, &watch->notifier);
-
-    xs_node_watch(xenbus->xsh, node, key, watch->token, &local_err);
+    xs_node_watch(watch_list->xsh, node, key, watch->token, &local_err);
     if (local_err) {
         error_propagate(errp, local_err);
 
@@ -181,18 +223,34 @@ static XenWatch *xen_bus_add_watch(XenBus *xenbus, const char *node,
     return watch;
 }
 
-static void xen_bus_remove_watch(XenBus *xenbus, XenWatch *watch,
+static void watch_list_remove(XenWatchList *watch_list, XenWatch *watch,
                               Error **errp)
 {
-    trace_xen_bus_remove_watch(watch->node, watch->key, watch->token);
-
-    xs_node_unwatch(xenbus->xsh, watch->node, watch->key, watch->token,
+    xs_node_unwatch(watch_list->xsh, watch->node, watch->key, watch->token,
                     errp);
 
     notifier_remove(&watch->notifier);
     free_watch(watch);
 }
 
+static XenWatch *xen_bus_add_watch(XenBus *xenbus, const char *node,
+                                   const char *key, XenWatchHandler handler,
+                                   void *opaque, Error **errp)
+{
+    trace_xen_bus_add_watch(node, key);
+
+    return watch_list_add(xenbus->watch_list, node, key, handler, opaque,
+                          errp);
+}
+
+static void xen_bus_remove_watch(XenBus *xenbus, XenWatch *watch,
+                                 Error **errp)
+{
+    trace_xen_bus_remove_watch(watch->node, watch->key);
+
+    watch_list_remove(xenbus->watch_list, watch, errp);
+}
+
 static void xen_bus_backend_create(XenBus *xenbus, const char *type,
                                    const char *name, char *path,
                                    Error **errp)
@@ -338,35 +396,14 @@ static void xen_bus_unrealize(BusState *bus, Error **errp)
         xenbus->backend_watch = NULL;
     }
 
-    if (!xenbus->xsh) {
-        return;
+    if (xenbus->watch_list) {
+        watch_list_destroy(xenbus->watch_list);
+        xenbus->watch_list = NULL;
     }
 
-    qemu_set_fd_handler(xs_fileno(xenbus->xsh), NULL, NULL, NULL);
-
+    if (xenbus->xsh) {
         xs_close(xenbus->xsh);
     }
-
-static void xen_bus_watch(void *opaque)
-{
-    XenBus *xenbus = opaque;
-    char **v;
-    const char *token;
-
-    g_assert(xenbus->xsh);
-
-    v = xs_check_watch(xenbus->xsh);
-    if (!v) {
-        return;
-    }
-
-    token = v[XS_WATCH_TOKEN];
-
-    trace_xen_bus_watch(token);
-
-    notifier_list_notify(&xenbus->watch_notifiers, (void *)token);
-
-    free(v);
 }
 
 static void xen_bus_realize(BusState *bus, Error **errp)
@@ -390,9 +427,7 @@ static void xen_bus_realize(BusState *bus, Error **errp)
         xenbus->backend_id = 0; /* Assume lack of node means dom0 */
     }
 
-    notifier_list_init(&xenbus->watch_notifiers);
-    qemu_set_fd_handler(xs_fileno(xenbus->xsh), xen_bus_watch, NULL,
-                        xenbus);
+    xenbus->watch_list = watch_list_create(xenbus->xsh);
 
     module_call_init(MODULE_INIT_XEN_BACKEND);
 

include/hw/xen/xen-bus.h

@@ -14,6 +14,7 @@
 
 typedef void (*XenWatchHandler)(void *opaque);
 
+typedef struct XenWatchList XenWatchList;
 typedef struct XenWatch XenWatch;
 typedef struct XenEventChannel XenEventChannel;
 
@@ -63,7 +64,7 @@ typedef struct XenBus {
     BusState qbus;
     domid_t backend_id;
     struct xs_handle *xsh;
-    NotifierList watch_notifiers;
+    XenWatchList *watch_list;
     XenWatch *backend_watch;
 } XenBus;
 

include/qemu/notify.h

@@ -40,6 +40,8 @@ void notifier_remove(Notifier *notifier);
 
 void notifier_list_notify(NotifierList *list, void *data);
 
+bool notifier_list_empty(NotifierList *list);
+
 /* Same as Notifier but allows .notify() to return errors */
 typedef struct NotifierWithReturn NotifierWithReturn;
 

util/notify.c

@@ -40,6 +40,11 @@ void notifier_list_notify(NotifierList *list, void *data)
     }
 }
 
+bool notifier_list_empty(NotifierList *list)
+{
+    return QLIST_EMPTY(&list->notifiers);
+}
+
 void notifier_with_return_list_init(NotifierWithReturnList *list)
 {
     QLIST_INIT(&list->notifiers);