Messages in this thread | | | Date | Fri, 10 Dec 2021 19:47:06 -0500 | Subject | Re: [PATCH] xen/gntdev: fix unmap notification order | From | Boris Ostrovsky <> |
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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>
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