xen/gntdev: fix unmap notification order

commit ce2f46f3531a03781181b7f4bd1ff9f8c5086e7e upstream.

While working with Xen's libxenvchan library I have faced an issue with
unmap notifications sent in wrong order if both UNMAP_NOTIFY_SEND_EVENT
and UNMAP_NOTIFY_CLEAR_BYTE were requested: first we send an event channel
notification and then clear the notification byte which renders in the below
inconsistency (cli_live is the byte which was requested to be cleared on unmap):

[  444.514243] gntdev_put_map UNMAP_NOTIFY_SEND_EVENT map->notify.event 6
libxenvchan_is_open cli_live 1
[  444.515239] __unmap_grant_pages UNMAP_NOTIFY_CLEAR_BYTE at 14

Thus it is not possible to reliably implement the checks like
- wait for the notification (UNMAP_NOTIFY_SEND_EVENT)
- check the variable (UNMAP_NOTIFY_CLEAR_BYTE)
because it is possible that the variable gets checked before it is cleared
by the kernel.

To fix that we need to re-order the notifications, so the variable is first
gets cleared and then the event channel notification is sent.
With this fix I can see the correct order of execution:

[   54.522611] __unmap_grant_pages UNMAP_NOTIFY_CLEAR_BYTE at 14
[   54.537966] gntdev_put_map UNMAP_NOTIFY_SEND_EVENT map->notify.event 6
libxenvchan_is_open cli_live 0

Cc: stable@vger.kernel.org
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20211210092817.580718-1-andr2000@gmail.com
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index fec1b65..59ffea8 100644 (file)
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -250,13 +250,13 @@ void gntdev_put_map(struct gntdev_priv *priv, struct gntdev_grant_map *map)
        if (!refcount_dec_and_test(&map->users))
                return;
 
+       if (map->pages && !use_ptemod)
+               unmap_grant_pages(map, 0, map->count);
+
        if (map->notify.flags & UNMAP_NOTIFY_SEND_EVENT) {
                notify_remote_via_evtchn(map->notify.event);
                evtchn_put(map->notify.event);
        }
-
-       if (map->pages && !use_ptemod)
-               unmap_grant_pages(map, 0, map->count);
        gntdev_free_map(map);
 }