linux-kernel - Re: [PATCH] xen/gntdev: fix unmap notification order

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Date:   Fri, 10 Dec 2021 19:47:06 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Oleksandr Andrushchenko <andr2000@...il.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:     jgross@...e.com, sstabellini@...nel.org,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] xen/gntdev: fix unmap notification order


On 12/10/21 4:28 AM, Oleksandr Andrushchenko wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.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@...r.kernel.org
> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>



Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>