linux-kernel - Re: [Xen-devel] [PATCH 2/3] xen/ring: Add a new macro to detect whether there is an overflow in requests and response.

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Date:	Fri, 25 Jan 2013 18:45:29 +0100
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Konrad Rzeszutek Wilk <konrad@...nel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 2/3] xen/ring: Add a new macro to detect whether
 there is an overflow in requests and response.

On 25/01/13 18:32, Konrad Rzeszutek Wilk wrote:
> We want to be able to exit if the difference between the request
> produced (what the frontend tells us) and the requests consumed
> (what we have so far processed) is greater than the ring size.
> 
> If so, we should terminate the loop as the request produced
> is not trusted and it means it is bogus.
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  include/xen/interface/io/ring.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/include/xen/interface/io/ring.h b/include/xen/interface/io/ring.h
> index 75271b9..1114dde 100644
> --- a/include/xen/interface/io/ring.h
> +++ b/include/xen/interface/io/ring.h
> @@ -188,6 +188,12 @@ struct __name##_back_ring {						\
>  #define RING_REQUEST_CONS_OVERFLOW(_r, _cons)				\
>      (((_cons) - (_r)->rsp_prod_pvt) >= RING_SIZE(_r))
>  
> +/* Loop termination condition: Is the difference between request produced
> + * and request consumed greater than the ring size. If so, terminate the
> + * loop. */
> +#define RING_REQUEST_PROD_OVERFLOW(_r, _cons, _prod)                     \
> +    (((_prod) - (_prod)) > RING_SIZE(_r))

Shouldn't the second _prod be _cons?

> +
>  #define RING_PUSH_REQUESTS(_r) do {					\
>      wmb(); /* back sees requests /before/ updated producer index */	\
>      (_r)->sring->req_prod = (_r)->req_prod_pvt;				\
> 

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