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

[Waiman Long]

On 07/22/2014 05:59 AM, Thomas Gleixner wrote:
> 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

I think NUMA futex, if implemented, is a completely independent piece 
that have no direct relationship with optimistic spinning futex. It 
should be a separate patch and not mixing with optimistic spinning patch 
which will only make the latter one more complicated.

-Longman
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