Re: [PATCH v2 16/18] libxl: introduce libxl_userdata_unlink

[Ian Campbell <ian.campbell@citrix.com>]

On Thu, 2014-08-28 at 12:50 +0100, Wei Liu wrote:
> On Wed, Aug 27, 2014 at 03:16:59AM +0100, Ian Campbell wrote:
> > On Wed, 2014-07-30 at 19:23 +0100, Wei Liu wrote:
> > > +    CTX_LOCK;
> > > +    lock = libxl__lock_domain_data(gc, domid);
> > > +    if (!lock) {
> > 
> > This seems like a quirk which deserves commenting on since this lock
> > only protects a specific type of userdata, specifically the libxl-json
> > userdata.
> > 
> 
> The original intent for that lock is to protect all application defined
> data -- the name in registry is "libxl-lock" not "libxl-json-lock".

libxl-lock suggests to me that it locks things internal to libxl, not
"application userdata lock".

> Though through out this series it's mostly used to protect libxl-json
> data, it doesn't preclude us from using it to protect other type of
> userdata.
> 

Except the locking functions aren't public, are they? and since the
interface uses carefd's I don't think it can be (easily) made public.

I guess what I'm saying is that if an app wants to lock accesses to its
userdata then it has to do that itself, it can't be done internally to
libxl.

> During review last round we discussed how we should deal with "xl
> config-udpate" command. The conclusion is that we still honour user
> supplied config file and it has higher priority than libxl-json. We
> would like to transform the config file supplied by user to libxl-json,
> then remove that user supplied file, so that next time domain is
> rebooted it always has the config tracked by libxl. Without this patch
> xl has no way to unlink that file and it will still take effect during
> next reboot, which is not what we want.

OK, so this is about removing the existing xl config, not the
libxl-json. I don't think this should take the libxl lock then -- that
lock doesn't protect the xl cfg userdata in any meaningful way AFAICT.

Ian.