linux-kernel - Re: [PATCH 9/9 v2] cgroup_freezer: implement proper hierarchy support

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Date:	Thu, 8 Nov 2012 16:57:48 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	lizefan@...wei.com, rjw@...k.pl,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	fweisbec@...il.com
Subject: Re: [PATCH 9/9 v2] cgroup_freezer: implement proper hierarchy support

On Thu 08-11-12 07:29:23, Tejun Heo wrote:
> Hey, Michal.
> 
> On Thu, Nov 08, 2012 at 04:20:39PM +0100, Michal Hocko wrote:
> > > So, in the above example in CPU2, (B->state & FREEZING) test and
> > > freezer_apply_state(C, false) can't be interleaved with the same
> > > inheritance operation from CPU1.  They either happen before or after.
> > 
> > I am not sure I understand what you mean by inheritance operation but
> 
> The operation of looking at one's parent and inherting the state.
> Here, checking parent->state and calling apply_state accordingly.
> 
> > you are right that the race is not possible because spin_lock makes
> > sure that Foo->state is done after the lock(Foo->child) and spin_unlock
> > then serves as a leave barrier so the other CPUs will see the correctly
> > updated value. The rest is handled by the fixed ordered tree walk. So
> > there is really no interleaving going on here.
> 
> I'm worried that this is a bit too subtle.

Dunno, it looks obvious now, I just missed the entry&leave implicit
barriers by spinlocks and again sorry about the confusion.

> This will work fine with a single hierarchy mutex protecting hierarchy
> updates and state propagations through it and that should work for
> most controllers too.

But single mutex is just ugly.

> I want to have at least one example of finer grained locking for
> future reference and cgroup_freezer happened to be the one I started
> working on.

I think this is a good example because it shows how to share the state
without too many implementation details.

> So, it is more complicated (not necessarily in written code but the
> sync rules) than necessary.  I'm still not sure whether to keep it or
> not.

I think the locking is fine and I would keep it this way rather than a
big lock.

> I'll add more documentation about synchronization in
> cgroup_for_each_descendant_pre() either way.

more doc cannot hurt ;)

Thanks
-- 
Michal Hocko
SUSE Labs
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