Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753015Ab1FGSn1 (ORCPT ); Tue, 7 Jun 2011 14:43:27 -0400 Received: from mail-pz0-f46.google.com ([209.85.210.46]:49954 "EHLO mail-pz0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752261Ab1FGSn0 convert rfc822-to-8bit (ORCPT ); Tue, 7 Jun 2011 14:43:26 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:sender:in-reply-to:references:from:date :x-google-sender-auth:message-id:subject:to:cc:content-type :content-transfer-encoding; b=mU/F3CB6XjbG9tLM2s0bSzJG8bs0n7iDwwiCTZJAp86LYdpc8qwgAOdwFJ2fUTiS0i j1cefjY6x8hreQshP/XR4qVRGTF1kjJVyidQJeGnwJ37EgyaujsMwaOKEfcxqnLARcvU Z4iOXRyVwsWOwQnUJzQKf1+lk1VULaJLd5wnI= MIME-Version: 1.0 In-Reply-To: <1307462300.3091.39.camel@edumazet-laptop> References: <1307373819.3098.40.camel@edumazet-laptop> <1307376672.2322.167.camel@twins> <1307376989.2322.171.camel@twins> <1307377349.3098.65.camel@edumazet-laptop> <1307377782.2322.183.camel@twins> <1307378564.3098.67.camel@edumazet-laptop> <4DED1421.5000300@linux.intel.com> <1307383898.3098.90.camel@edumazet-laptop> <4DED976C.90009@linux.intel.com> <4DEE3944.5020005@mit.edu> <1307462300.3091.39.camel@edumazet-laptop> From: Andrew Lutomirski Date: Tue, 7 Jun 2011 14:43:06 -0400 X-Google-Sender-Auth: NxDQzIO7YlL5snSLGKnSnL4sBd4 Message-ID: Subject: Re: Change in functionality of futex() system call. To: Eric Dumazet Cc: Darren Hart , Peter Zijlstra , David Oliver , linux-kernel@vger.kernel.org, Shawn Bohrer , Zachary Vonler , KOSAKI Motohiro , Hugh Dickins , Thomas Gleixner , Ingo Molnar Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2852 Lines: 72 On Tue, Jun 7, 2011 at 11:58 AM, Eric Dumazet wrote: > Le mardi 07 juin 2011 ? 10:44 -0400, Andy Lutomirski a ?crit : >> On 06/06/2011 11:13 PM, Darren Hart wrote: >> > >> > >> > On 06/06/2011 11:11 AM, Eric Dumazet wrote: >> >> Le lundi 06 juin 2011 ? 10:53 -0700, Darren Hart a ?crit : >> >>> >> >> >> >>> If I understand the problem correctly, RO private mapping really doesn't >> >>> make any sense and we should probably explicitly not support it, while >> >>> special casing the RO shared mapping in support of David's scenario. >> >>> >> >> >> >> We supported them in 2.6.18 kernels, apparently. This might sounds >> >> stupid but who knows ? >> > >> > >> > I guess this is actually the key point we need to agree on to provide a >> > solution. This particular case "worked" in 2.6.18 kernels, but that >> > doesn't necessarily mean it was supported, or even intentional. >> > >> > It sounds to me that we agree that we should support RO shared mappings. >> > The question remains about whether we should introduce deliberate >> > support of RO private mappings, and if so, if the forced COW approach is >> > appropriate or not. >> > >> >> I disagree. >> >> FUTEX_WAIT has side-effects. ?Specifically, it eats one wakeup sent by >> FUTEX_WAKE. ?So if something uses futexes on a file mapping, then a >> process with only read access could (if the semantics were changed) DoS >> the other processes by spawning a bunch of threads and FUTEX_WAITing >> from each of them. >> >> If there were a FUTEX_WAIT_NOCONSUME that did not consume a wakeup and >> worked on RO mappings, I would drop my objection. > > If a group of cooperating processes uses a memory segment to exchange > critical information, do you really think this memory segment will be > readable by other unrelated processes on the machine ? Depends on the design. I have some software I'm working on that uses shared files and could easily use futexes. I don't want random read-only processes to interfere with the futex protocol. > > How is this related to futex code ? Because this usage is currently safe and would become unsafe with the proposed change. > > Same problem for legacy IPC (shm, msg, sem) : Appropriate protections > are needed, obviously. > > BTW, kernel/futex.c uses a global hash table (futex_queues[256]) and a > very predictable hash_futex(), so its easy to slow down futex users... There's a difference between slowing down users by abusing a kernel hash and deadlocking users by eating a wakeup. (If you eat a wakeup the wakeup won't magically come back later. It's gone.) --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/