Subject: Re: [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant read_barrier_depends()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 5 Dec 2017 20:31:20 +0200

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 <[email-masked]>
> Cc: "Michael S. Tsirkin" <[email-masked]>
> Cc: Jason Wang <[email-masked]>
> Cc: <[email-masked]>
> Cc: <[email-masked]>
> Cc: <[email-masked]>

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