linux-kernel - Re: [PATCH 3/3] mm/memcg: iteration skip memcgs not yet fully initialized

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Message-ID: <20140115082058.GA8782@dhcp22.suse.cz>
Date:	Wed, 15 Jan 2014 09:20:58 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] mm/memcg: iteration skip memcgs not yet fully
 initialized

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
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