Re: [PATCH 3/9] bio-cgroup controller

[Ryo Tsuruta]

Hi Andrea,

> On Mon, Apr 20, 2009 at 08:35:40PM +0900, Ryo Tsuruta wrote:
> > > > +/*
> > > > + * Assign "page" the same owner as "opage."
> > > > + */
> > > > +void bio_cgroup_copy_owner(struct page *npage, struct page *opage)
> > > > +{
> > > > +	struct page_cgroup *npc, *opc;
> > > > +
> > > > +	if (bio_cgroup_disabled())
> > > > +		return;
> > > > +	npc = lookup_page_cgroup(npage);
> > > > +	if (unlikely(!npc))
> > > > +		return;
> > > > +	opc = lookup_page_cgroup(opage);
> > > > +	if (unlikely(!opc))
> > > > +		return;
> > > > +
> > > > +	/*
> > > > +	 * Do this without any locks. The reason is the same as
> > > > +	 * bio_cgroup_reset_owner().
> > > > +	 */
> > > > +	npc->bio_cgroup_id = opc->bio_cgroup_id;
> > > 
> > > What protects npc and opc?
> > 
> > As the same reason mentioned above, bio_cgroup_id can be updated
> > without any locks, and npc and opc always point to page_cgroups.
> > An integer variable can be set a new value at once on a system which
> > can use RCU lock.
> 
> mmmh... I'm not sure about this. Actually you read opc->bio_cgroup_id
> first and then write to npc->bio_cgroup_id, so it is not atomic at all.
> So, you can read or set a wrong ID, but at least it should be always
> consistent (the single read or write itself is atomic).

Even if opc->bio_cgroup_id is changed before copying to
npc->bio_cgroup_id, npc->bio_cgroup_id is correctly updated at some time.
The implementation is not completely accurate, but is faster and lighter.

Thanks,
Ryo Tsuruta
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