Re: [PATCH 3 of 5] Rework locking in the PoD layer

[Tim Deegan <tim@xen.org>]

At 14:56 -0500 on 01 Feb (1328108167), Andres Lagar-Cavilla wrote:
>  xen/arch/x86/mm/mm-locks.h |   10 ++++
>  xen/arch/x86/mm/p2m-pod.c  |  112 ++++++++++++++++++++++++++------------------
>  xen/arch/x86/mm/p2m-pt.c   |    1 +
>  xen/arch/x86/mm/p2m.c      |    8 ++-
>  xen/include/asm-x86/p2m.h  |   27 +++-------
>  5 files changed, 93 insertions(+), 65 deletions(-)
> 
> 
> The PoD layer has a comples locking discipline. It relies on the
> p2m being globally locked, and it also relies on the page alloc
> lock to protect some of its data structures. Replace this all by an
> explicit pod lock: per p2m, order enforced.
> 
> Three consequences:
>     - Critical sections in the pod code protected by the page alloc
>       lock are now reduced to modifications of the domain page list.
>     - When the p2m lock becomes fine-grained, there are no
>       assumptions broken in the PoD layer.
>     - The locking is easier to understand.
> 
> Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>

This needs an Ack from George, too.  Also:

> @@ -922,6 +929,12 @@ p2m_pod_emergency_sweep(struct p2m_domai
>      limit = (start > POD_SWEEP_LIMIT) ? (start - POD_SWEEP_LIMIT) : 0;
>  
>      /* FIXME: Figure out how to avoid superpages */
> +    /* NOTE: Promote to globally locking the p2m. This will get complicated
> +     * in a fine-grained scenario. Even if we're to lock each gfn 
> +     * individually we must be careful about recursion limits and 
> +     * POD_SWEEP_STRIDE. This is why we don't enforce deadlock constraints 
> +     * between p2m and pod locks */
> +    p2m_lock(p2m);

That's a scary comment.  It looks to me as if the mm-locks.h mechanism
_does_ enforce those constraints - am I missing something?

Cheers,

Tim.