WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-devel

Re: [Xen-devel] One shared memory read/write issue between 2 different d

To: <chunjie_zhu@xxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] One shared memory read/write issue between 2 different domains
From: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Date: Mon, 6 Jun 2011 10:10:01 +0100
Cc: Jianxin_Guo@xxxxxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxx
Delivery-date: Mon, 06 Jun 2011 02:11:23 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <13B81B9DDD69EC4DB30F266136B6832501393FCD@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <13B81B9DDD69EC4DB30F266136B6832501393FCD@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Mutt/1.5.21 (2010-09-15)
Hi, 

At 17:39 +0800 on 25 May (1306345173), chunjie_zhu@xxxxxxxxxxxxxxxxx wrote:
> I code one ring buffer by myself for 2 domains to transmit data, one
> domain is producer, and the other is consumer. I do not use the ring
> buffer defined in ??include/xen/interface/io/ring.h??.

Why not?  That one has had a lot of testing and debugging.

> Just like the ring buffer provided by xen, I also use 2 pointers for
> producer and consumer, separately. To ensure memory access order is
> consistent, I use memory barrier between memory store/load operation
> and pointer update operation.
> 
> The transmitted data contains 2 parts, one is header, and the other is
> payload. Each time, one domain transmits the header first, and then
> the payload is sent out.
> 
> Each time I can get right header at the consumer side, however,
> sometimes, the data I get at the consumer side contains part of
> invalid information.
> 
> Could you please give me some suggestion on this issue?

Hard to say from just this description, but probably you're missing
either a hardware memory barrier or a compiler one.   Either that or
something in the rest of your code is corrupting the rings. 

Try comparing your code very carefully to the existing, working ring
buffer code.

> [Other Questions]
> 
> 1.  Is it reasonable to use spin lock to protect shared memory between
> 2 different domains?

It's probably not a great idea.  You have to worry about what happens if
one of the domains crashes while holding the lock, for example.

> 2.  Previously I use one atomic variable to do the synchronization
> between producer and consumer, however, sometimes I cannot get correct
> header at the consumer side. Is it one known-issue?

Again, you're probably missing a compiler barrier.  AIUI, updates to
atomic variables are only guaranteed not to be reordered WRT other write
of the _same_ variable.

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel

<Prev in Thread] Current Thread [Next in Thread>