linux-kernel - Re: [PATCH 0/3] virtio/vringh: kill off ACCESS

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Date:   Thu, 24 Nov 2016 22:36:58 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, dave@...olabs.net, dbueso@...e.de,
        dvyukov@...gle.com, jasowang@...hat.com, kvm@...r.kernel.org,
        netdev@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()

On Thu, Nov 24, 2016 at 10:25:11AM +0000, Mark Rutland wrote:
> For several reasons, it would be beneficial to kill off ACCESS_ONCE()
> tree-wide, in favour of {READ,WRITE}_ONCE(). These work with aggregate types,
> more obviously document their intended behaviour, and are necessary for tools
> like KTSAN to work correctly (as otherwise reads and writes cannot be
> instrumented separately).
> 
> While it's possible to script the bulk of this tree-wide conversion, some cases
> such as the virtio code, require some manual intervention. This series moves
> the virtio and vringh code over to {READ,WRITE}_ONCE(), in the process fixing a
> bug in the virtio headers.
> 
> Thanks,
> Mark.

I don't have a problem with this specific patchset.

Though I really question the whole _ONCE APIs esp with
aggregate types - these seem to generate a memcpy and
an 8-byte read/writes sometimes, and I'm pretty sure this simply
can't be read/written at once on all architectures.

So I worry it's kind of like volatile in this respect,
too easy to overuse.


> Mark Rutland (3):
>   tools/virtio: fix READ_ONCE()
>   vringh: kill off ACCESS_ONCE()
>   tools/virtio: use {READ,WRITE}_ONCE() in uaccess.h
> 
>  drivers/vhost/vringh.c        | 5 +++--
>  tools/virtio/linux/compiler.h | 2 +-
>  tools/virtio/linux/uaccess.h  | 9 +++++----
>  3 files changed, 9 insertions(+), 7 deletions(-)
> 
> -- 
> 2.7.4