Re: [PATCH 1/2] libceph: add a compatibility check interface

[Josh Durgin <josh.durgin@inktank.com>]

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

On 01/30/2013 12:49 PM, Alex Elder wrote:
> An upcoming change implements semantic change that could lead to
> a crash if an old version of the libceph kernel module is used with
> a new version of the rbd kernel module.
>
> In order to preclude that possibility, this adds a compatibilty
> check interface.  If this interface doesn't exist, the modules are
> obviously not compatible.  But if it does exist, this provides a way
> of letting the caller know whether it will operate properly with
> this libceph module.
>
> Perhaps confusingly, it returns false right now.  The semantic
> change mentioned above will make it return true.
>
> This resolves:
>      http://tracker.ceph.com/issues/3800
>
> Signed-off-by: Alex Elder <elder@inktank.com>
> ---
>   include/linux/ceph/libceph.h |    2 ++
>   net/ceph/ceph_common.c       |   16 ++++++++++++++++
>   2 files changed, 18 insertions(+)
>
> diff --git a/include/linux/ceph/libceph.h b/include/linux/ceph/libceph.h
> index 084d3c6..c44275a 100644
> --- a/include/linux/ceph/libceph.h
> +++ b/include/linux/ceph/libceph.h
> @@ -193,6 +193,8 @@ static inline int calc_pages_for(u64 off, u64 len)
>   }
>
>   /* ceph_common.c */
> +extern bool libceph_compatible(void *data);
> +
>   extern const char *ceph_msg_type_name(int type);
>   extern int ceph_check_fsid(struct ceph_client *client, struct ceph_fsid
> *fsid);
>   extern struct kmem_cache *ceph_inode_cachep;
> diff --git a/net/ceph/ceph_common.c b/net/ceph/ceph_common.c
> index ee71ea2..a98c03f 100644
> --- a/net/ceph/ceph_common.c
> +++ b/net/ceph/ceph_common.c
> @@ -26,6 +26,22 @@
>   #include "crypto.h"
>
>
> +/*
> + * Module compatibility interface.  For now it doesn't do anything,
> + * but its existence signals a certain level of functionality.
> + *
> + * The data buffer is used to pass information both to and from
> + * libceph.  The return value indicates whether libceph determines
> + * it is compatible with the caller (from another kernel module),
> + * given the provided data.
> + *
> + * The data pointer can be null.
> + */
> +bool libceph_compatible(void *data)
> +{
> +	return false;
> +}
> +EXPORT_SYMBOL(libceph_compatible);
>
>   /*
>    * find filename portion of a path (/foo/bar/baz -> baz)
>