Re: [Qemu-devel] [PATCH 3/3] xen: perform XenDevice clean-up in XenBus watch handler

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Paul Durrant <paul.durrant@citrix.com>
> Sent: 11 September 2019 15:36
> To: qemu-devel@nongnu.org; xen-devel@lists.xenproject.org
> Cc: Paul Durrant <Paul.Durrant@citrix.com>; Stefano Stabellini <sstabellini@kernel.org>; Anthony
> Perard <anthony.perard@citrix.com>
> Subject: [PATCH 3/3] xen: perform XenDevice clean-up in XenBus watch handler
> 
> Cleaning up offine 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 'offline_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 'offline_devices' and perform the necessary actions.
> For safety a 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 include 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>
> ---
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Cc: Anthony Perard <anthony.perard@citrix.com>
> ---
>  hw/xen/trace-events      |  2 +
>  hw/xen/xen-bus.c         | 91 +++++++++++++++++++++++++++++-----------
>  include/hw/xen/xen-bus.h |  2 +
>  3 files changed, 70 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..055beb7260 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 +389,44 @@ 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->offline_devices, list, next) {
> +        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 +475,7 @@ 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 +597,9 @@ 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 +636,29 @@ 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 'offline' 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)) {
> +        XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
> 
> -        if (!xen_backend_try_device_destroy(xendev, &local_err)) {
> -            object_unparent(OBJECT(xendev));
> -        }
> +        QLIST_INSERT_HEAD(&xenbus->offline_devices, xendev, list);

I realize now that I should have a check here to make sure the above does not happen twice. I'll send a v2.

  Paul

> 
> -        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..15d71aff70 100644
> --- a/include/hw/xen/xen-bus.h
> +++ b/include/hw/xen/xen-bus.h
> @@ -33,6 +33,7 @@ typedef struct XenDevice {
>      xengnttab_handle *xgth;
>      bool feature_grant_copy;
>      QLIST_HEAD(, XenEventChannel) event_channels;
> +    QLIST_ENTRY(XenDevice) list;
>  } XenDevice;
> 
>  typedef char *(*XenDeviceGetName)(XenDevice *xendev, Error **errp);
> @@ -68,6 +69,7 @@ typedef struct XenBus {
>      struct xs_handle *xsh;
>      XenWatchList *watch_list;
>      XenWatch *backend_watch;
> +    QLIST_HEAD(, XenDevice) offline_devices;
>  } XenBus;
> 
>  typedef struct XenBusClass {
> --
> 2.20.1.2.gb21ebb6