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authorOleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>2021-12-10 12:28:17 +0300
committerJuergen Gross <jgross@suse.com>2022-01-06 10:52:22 +0300
commitce2f46f3531a03781181b7f4bd1ff9f8c5086e7e (patch)
tree6aa5aea0cea2cad66e46d7c565afefa7d96d9896 /drivers/xen
parentc9e6606c7fe92b50a02ce51dda82586ebdf99b48 (diff)
downloadlinux-ce2f46f3531a03781181b7f4bd1ff9f8c5086e7e.tar.xz
xen/gntdev: fix unmap notification order
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>
Diffstat (limited to 'drivers/xen')
-rw-r--r--drivers/xen/gntdev.c6
1 files changed, 3 insertions, 3 deletions
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index fec1b6537166..59ffea800079 100644
--- 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);
}