linux-kernel - Re: Attaching a process to cgroups

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Date:	Wed, 27 Jun 2012 10:10:44 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mike Galbraith <efault@....de>
Cc:	Alexey Vlasov <renton@...ton.name>, linux-kernel@...r.kernel.org
Subject: Re: Attaching a process to cgroups

On Wed, Jun 27, 2012 at 09:23:31AM +0200, Mike Galbraith wrote:
> On Tue, 2012-06-26 at 11:06 -0700, Paul E. McKenney 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 ;-)
> > 
> > Or replace it with synchronize_rcu_expedited().  You can "get lucky"
> > for quite some time removing synchronize_rcu() calls!
> 
> s/remove/replace, but yup.  A company that wanted to use the android
> patches plus my tinkering showed a fix they needed on top to close a
> race discovered in their testing.  So yeah, even when all seems fine,
> extracting synchronize_rcu() may expose evils you couldn't encounter
> before, and didn't happen to encounter afterward.

I really did mean "remove".  Removing a synchronize_rcu() does result
in a race, but often an extremely low-probability race.  So you can
remove a synchronize_rcu() and get lucky for a long time, but sooner
or later, something will explode.

							Thanx, Paul

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