From: Oleksandr Andrushchenko oleksandr_andrushchenko@epam.com
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 --- drivers/xen/gntdev.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)
--- a/drivers/xen/gntdev.c +++ b/drivers/xen/gntdev.c @@ -240,13 +240,13 @@ void gntdev_put_map(struct gntdev_priv * 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); }