Messages in this thread | | | Date | Tue, 22 Jul 2014 16:25:16 -0400 | From | Waiman Long <> | Subject | Re: [RFC PATCH 0/5] futex: introduce an optimistic spinning futex |
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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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