Subject: Re: [PATCH v3 07/19] vhost: Remove redundant use of read_barrier_depends() barrier

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 13 Jul 2020 07:27:01 -0400

On Fri, Jul 10, 2020 at 05:51:51PM +0100, Will Deacon wrote:
> Since commit 76ebbe78f739 ("locking/barriers: Add implicit
> smp_read_barrier_depends() to READ_ONCE()"), there is no need to use
> smp_read_barrier_depends() outside of the Alpha architecture code.
> 
> Unfortunately, there is precisely _one_ user in the vhost code, and
> there isn't an obvious READ_ONCE() access making the barrier
> redundant. However, on closer inspection (thanks, Jason), it appears
> that vring synchronisation between the producer and consumer occurs via
> the 'avail_idx' field, which is followed up by an rmb() in
> vhost_get_vq_desc(), making the read_barrier_depends() redundant on
> Alpha.
> 
> Jason says:
> 
>   | I'm also confused about the barrier here, basically in driver side
>   | we did:
>   |
>   | 1) allocate pages
>   | 2) store pages in indirect->addr
>   | 3) smp_wmb()
>   | 4) increase the avail idx (somehow a tail pointer of vring)
>   |
>   | in vhost we did:
>   |
>   | 1) read avail idx
>   | 2) smp_rmb()
>   | 3) read indirect->addr
>   | 4) read from indirect->addr
>   |
>   | It looks to me even the data dependency barrier is not necessary
>   | since we have rmb() which is sufficient for us to the correct
>   | indirect->addr and driver are not expected to do any writing to
>   | indirect->addr after avail idx is increased
> 
> Remove the redundant barrier invocation.
> 
> Suggested-by: Jason Wang <[email-masked]>
> Acked-by: Paul E. McKenney <[email-masked]>
> Signed-off-by: Will Deacon <[email-masked]>

I agree

Acked-by: Michael S. Tsirkin <[email-masked]>

Pls merge with the rest of the patchset.

> ---
>  drivers/vhost/vhost.c | 5 -----
>  1 file changed, 5 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index d7b8df3edffc..74d135ee7e26 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2092,11 +2092,6 @@ static int get_indirect(struct vhost_virtqueue *vq,
>  		return ret;
>  	}
>  	iov_iter_init(&from, READ, vq->indirect, ret, len);
> -
> -	/* We will use the result as an address to read from, so most
> -	 * architectures only need a compiler barrier here. */
> -	read_barrier_depends();
> -
>  	count = len / sizeof desc;
>  	/* Buffers are chained via a 16 bit next field, so
>  	 * we can have at most 2^16 of these. */
> -- 
> 2.27.0.383.g050319c2ae-goog

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