Re: Atomic operations

[Juergen Gross <juergen.gross@fujitsu-siemens.com>]

Keir Fraser wrote:
> The Linux atomic.h operations will be atomic even across domains.

... unless CONFIG_SMP was not defined in the domU kernel build.


Juergen

> On 26/03/2009 00:17, "Timothy Hayes" <hayesti@tcd.ie> wrote:
> 
>     I'm sure a lot of you know XenSocket (XVMSocket), if not, it's a
>     Loadable Kernel Module with a socket interface that sends/receives
>     to/from a shared page of memory mapped between two domains. It's
>     quite a nice idea and brings data throughput close to UNIX sockets.
>     I made some changes and made it compatible with Xen 3.2 and a newer
>     Linux kernel (I'm testing it with 2.6.25.20) however I'm noticing
>     some odd behaviour occasionally. In it, there are atomic_t variables
>     in a descriptor inside a shared memory page. When a domain sends or
>     receives from the socket, it will use one of the atomic_ operations
>     to update the amount of new data available. The problem I'm noticing
>     is that when two domains are concurrently updating this variable,
>     the value becomes inconsistent leading me to believe that the
>     atomic_ operations might not guarantee atomicity between domains. I
>     haven't been able to repeat any behaviour like this when I limit the
>     two guests to the same physical CPU and eliminate parallelism.
> 
>     Would anyone be able fill me in if I'm missing something? Are the
>     atomic_ operations 100% foolproof for this sort of thing? If not,
>     what would be the best approach to use instead?
> 
> 
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Juergen Gross                             Principal Developer
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