On Tue 14-01-14 09:29:04, Tejun Heo wrote: > Hey, Michal. > > On Tue, Jan 14, 2014 at 02:30:05PM +0100, Michal Hocko wrote: > > On Mon 13-01-14 17:54:04, Hugh Dickins wrote: > > > It is surprising that the mem_cgroup iterator can return memcgs which > > > have not yet been fully initialized. By accident (or trial and error?) > > > this appears not to present an actual problem; but it may be better to > > > prevent such surprises, by skipping memcgs not yet online. > > > > My understanding was that !online cgroups are not visible for the > > iterator. it is css_online that has to be called before they are made > > visible. > > > > Tejun? > > From the comment above css_for_each_descendant_pre() > > * Walk @root's descendants. @root is included in the iteration and the > * first node to be visited. Must be called under rcu_read_lock(). A > * descendant css which hasn't finished ->css_online() or already has > * finished ->css_offline() may show up during traversal and it's each > * subsystem's responsibility to verify that each @pos is alive. /me slaps self. I was even reviewing patches which introduced that. But still I managed to convince myself that online means before online rather than right after again and again. Sorry about the confusion. > What it guarantees is that an online css would *always* show up in the > iteration. It's kinda difficult to guarantee both directions just > with RCU locking. You gotta make at least one end loose to make it > work with RCU. > > Thanks. > > -- > tejun -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>