linux-kernel - Re: Attaching a process to cgroups

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Date:	Tue, 24 Jul 2012 03:19:15 +0200
From:	Mike Galbraith <efault@....de>
To:	Andrea Righi <righi.andrea@...il.com>
Cc:	Alexey Vlasov <renton@...ton.name>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Attaching a process to cgroups

On Mon, 2012-07-23 at 22:41 +0200, Andrea Righi wrote: 
> On Thu, Jun 21, 2012 at 10:23:02AM +0200, Mike Galbraith wrote:
> > On Thu, 2012-06-21 at 11:54 +0400, Alexey Vlasov wrote: 
> > > On Wed, Jun 20, 2012 at 02:28:18PM +0200, Mike Galbraith wrote:
> > > > 
> > > > kernel/cgroup.c::cgroup_attach_task()
> > > > {
> > > > ...
> > > > 	synchronize_rcu();
> > > > ...
> > > > }
> > > 
> > > So nothing can be done here? (I mean if only I knew how to fix it I
> > > wouldn't ask about it ;)
> > 
> > Sure, kill the obnoxious thing, it's sitting right in the middle of the
> > userspace interface.
> > 
> > I banged on it a while back (wrt explosive android patches), extracted
> > RCU from the userspace interface.  It seemed to work great, much faster,
> > couldn't make it explode.  I wouldn't bet anything I wasn't willing to
> > immediately part with that the result was really really safe though ;-)
> > 
> > -Mike
> 
> JFYI,
> 
> I'm testing the following patch in a bunch of hosts and I wasn't able to
> make any of them to explode, even running a multi-threaded
> cgroup-intensive workload, but probably I was just lucky (or unlucky,
> depending on the point of view).
> 
> It is basically the same Not-signed-off-by work posted by Mike a while
> ago: https://lkml.org/lkml/2011/4/12/599.
> 
> In addition, I totally removed the synchronize_rcu() call from
> cgroup_attach_task() and added the call_rcu -> schedule_work removal
> also for css_set. The latter looks unnecessary to me from a logical
> point of view, or maybe I'm missing something, because I can't explain
> why with it I can't trigger any BUG / oops.
> 
> Mike, did you make any progress from your old patch?

No, it worked, but I couldn't prove it was really safe, so let it drop.

-Mike

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