drivers/xen/gntdev.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)
From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
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>
---
drivers/xen/gntdev.c | 6 +++---
1 file 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);
}
--
2.25.1
On 12/10/21 4:28 AM, Oleksandr Andrushchenko wrote: > From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com> > > 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>
On 12/10/21 4:28 AM, Oleksandr Andrushchenko wrote: > From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com> > > 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> Applied to for-linus-5.16c -boris
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