Re: [dpdk-dev] [PATCH] vhost: fix double read of descriptor flags

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 12/5/18 4:09 PM, Ilya Maximets wrote:
> Flags could be updated in a separate process leading to the
> inconsistent check.
> 
> Additionally, read marked as 'volatile' to highlight the shared
> nature of the variable and avoid such issues in the future.
> 
> Fixes: d3211c98c456 ("vhost: add helpers for packed virtqueues")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
> ---
>   lib/librte_vhost/vhost.h | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
> index 5218f1b12..84cbee2b6 100644
> --- a/lib/librte_vhost/vhost.h
> +++ b/lib/librte_vhost/vhost.h
> @@ -393,8 +393,10 @@ vq_is_packed(struct virtio_net *dev)
>   static inline bool
>   desc_is_avail(struct vring_packed_desc *desc, bool wrap_counter)
>   {
> -	return wrap_counter == !!(desc->flags & VRING_DESC_F_AVAIL) &&
> -		wrap_counter != !!(desc->flags & VRING_DESC_F_USED);
> +	uint16_t flags = *((volatile uint16_t *) &desc->flags);
> +
> +	return wrap_counter == !!(flags & VRING_DESC_F_AVAIL) &&
> +		wrap_counter != !!(flags & VRING_DESC_F_USED);
>   }
>   
>   #define VHOST_LOG_PAGE	4096
> 

Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>

Thanks,
Maxime