Re: [RFC PATCH 0/5] futex: introduce an optimistic spinning futex

Thomas Gleixner
2014-07-22T10:00:25Z

On Tue, 22 Jul 2014, Peter Zijlstra wrote:
> On Tue, Jul 22, 2014 at 10:39:17AM +0200, Thomas Gleixner wrote:
> > On Tue, 22 Jul 2014, Peter Zijlstra wrote:
> > > Anyway, there is one big fail in the entire futex stack that we 'need'
> > > to sort some day and that is NUMA. Some people (again database people)
> > > explicitly do not use futexes and instead use sysvsem because of this.
> > >
> > > The problem with numa futexes is that because they're vaddr based there
> > > is no (persistent) node information. You always end up having to fall
> > > back to looking in all nodes before you can guarantee there is no
> > > matching futex.
> > >
> > > One way to achieve it is by extending the futex value to include a node
> > > number, but that's obviously a complete ABI break. Then again, it should
> > > be pretty straight fwd, since the node number doesn't need to be part of
> > > the actual atomic update part, just part of the userspace storage.
> >
> > So you want per node hash buckets, right? Fair enough, but how do you
> > make sure, that no thread/process on a different node is fiddling with
> > that "node bound" futex as well?
>
> You don't and that should work just as well, just slower. But since the
> node id is in the futex 'value' we'll always end up in the right
> node-hash, even if its a remote one.
>
> So yes, per node hashes, and a persistent futex->node map.

Which works fine as long as you only have the futex_q on the stack of
the blocked task. If user space is lying to you, then you just end up
with a bunch of threads sleeping forever. Who cares?

But if you create independent kernel state, which we have with
pi_state and which you need for finegrained locking and further
spinning fun, you open up another can of worms. Simply because this
would enable rogue user space to create multiple instances of the
kernel internal state. I can predict the CVEs resulting from that
even without using a crystal ball.

Thanks,

tglx