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SubjectRe: 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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