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author | Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com> | 2021-12-10 11:28:17 +0200 |
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committer | Boris Ostrovsky <boris.ostrovsky@oracle.com> | 2021-12-10 18:54:29 -0600 |
commit | bc3bfd76d55ff57d648e65f1a2308ac6f9e6f14b (patch) | |
tree | 45964c39b04d515493192eb0d899d34a00ae9a94 | |
parent | 00db58cf21188f4b99bc5f15fcc2995e30e4a9fe (diff) | |
download | linux-for-linus-5.16c.tar.gz |
xen/gntdev: fix unmap notification orderfor-linus-5.16c
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: Boris Ostrovsky <boris.ostrovsky@oracle.com>
-rw-r--r-- | drivers/xen/gntdev.c | 6 |
1 files changed, 3 insertions, 3 deletions
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c index fec1b653716650..59ffea80007914 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); } |