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SubjectRe: [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant read_barrier_depends()
On Fri, Dec 01, 2017 at 11:51:16AM -0800, Paul E. McKenney wrote:
> Because READ_ONCE() now implies read_barrier_depends(), the
> read_barrier_depends() in next_desc() is now redundant. This commit
> therefore removes it and the related comments.
>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Jason Wang <jasowang@redhat.com>
> Cc: <kvm@vger.kernel.org>
> Cc: <virtualization@lists.linux-foundation.org>
> Cc: <netdev@vger.kernel.org>

Apropos, READ_ONCE is now asymmetrical with WRITE_ONCE.

I can read a pointer with READ_ONCE and be sure the value
is sane, but only if I also remember to put in smp_wmb before
WRITE_ONCE. Otherwise the pointer is ok but no guarantees
about the data pointed to.

It would be better if the API reflected he assymetry somehow.

> ---
> drivers/vhost/vhost.c | 7 +------
> 1 file changed, 1 insertion(+), 6 deletions(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 33ac2b186b85..78b5940a415a 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1877,12 +1877,7 @@ static unsigned next_desc(struct vhost_virtqueue *vq, struct vring_desc *desc)
> return -1U;
>
> /* Check they're not leading us off end of descriptors. */
> - next = vhost16_to_cpu(vq, desc->next);
> - /* Make sure compiler knows to grab that: we don't want it changing! */
> - /* We will use the result as an index in an array, so most
> - * architectures only need a compiler barrier here. */
> - read_barrier_depends();
> -
> + next = vhost16_to_cpu(vq, READ_ONCE(desc->next));
> return next;
> }
>
> --
> 2.5.2

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