Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754115Ab1FGP60 (ORCPT ); Tue, 7 Jun 2011 11:58:26 -0400 Received: from mail-wy0-f174.google.com ([74.125.82.174]:58141 "EHLO mail-wy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753934Ab1FGP6Z (ORCPT ); Tue, 7 Jun 2011 11:58:25 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=subject:from:to:cc:in-reply-to:references:content-type:date :message-id:mime-version:x-mailer:content-transfer-encoding; b=t9/wrb7OY8kb6w/F3DfkUa8MtntsPTwjHF3OUoL8jhi5wxFwGBKJwQPVTwOSNIjScN 8mEfvy0eYFhxNmrqgWBkgfxcZsihqaNt7Y9bLs+g5iiw3sy4G5prThZ3QBNqF8bfGvIu wl7YReblu7gaIQ8rw5+ZTZx4JMH0/aAPh9aYo= Subject: Re: Change in functionality of futex() system call. From: Eric Dumazet To: Andy Lutomirski Cc: Darren Hart , Peter Zijlstra , David Oliver , linux-kernel@vger.kernel.org, Shawn Bohrer , Zachary Vonler , KOSAKI Motohiro , Hugh Dickins , Thomas Gleixner , Ingo Molnar In-Reply-To: <4DEE3944.5020005@mit.edu> 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> Content-Type: text/plain; charset="UTF-8" Date: Tue, 07 Jun 2011 17:58:20 +0200 Message-ID: <1307462300.3091.39.camel@edumazet-laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.32.2 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2246 Lines: 57 Le mardi 07 juin 2011 à 10:44 -0400, Andy Lutomirski a écrit : > 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 à 10:53 -0700, Darren Hart a écrit : > >>> > >> > >>> 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 ? How is this related to futex code ? 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... -- 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/