[PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()

Michael S. Tsirkin
Thu, 24 Nov 2016 20:36:58 -0000

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