From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [RFC PATCH 0/5] futex: introduce an optimistic spinning futex Date: Tue, 22 Jul 2014 16:25:16 -0400 Message-ID: <53CEC8AC.7020700@hp.com> References: <20140721212740.GS3935@laptop> <20140721213457.46623e2f@gandalf.local.home> <20140722074719.GV3935@laptop> <20140722084842.GZ3935@laptop> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-doc-owner@vger.kernel.org To: Thomas Gleixner Cc: Peter Zijlstra , Steven Rostedt , Darren Hart , Andy Lutomirski , Andi Kleen , Ingo Molnar , Davidlohr Bueso , Heiko Carstens , "linux-kernel@vger.kernel.org" , Linux API , "linux-doc@vger.kernel.org" , Jason Low , Scott J Norton , Robert Haas List-Id: linux-api@vger.kernel.org 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