Messages in this thread |  | | | Date | Mon, 20 Apr 2009 16:56:59 +0200 | | From | Andrea Righi <> | | Subject | Re: [PATCH 3/9] bio-cgroup controller |
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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).
-Andrea
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