From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 49A03C433E0 for ; Wed, 3 Jun 2020 14:34:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2CE9820679 for ; Wed, 3 Jun 2020 14:34:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726154AbgFCOeO (ORCPT ); Wed, 3 Jun 2020 10:34:14 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57186 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725834AbgFCOeN (ORCPT ); Wed, 3 Jun 2020 10:34:13 -0400 Received: from Galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BC207C08C5C0; Wed, 3 Jun 2020 07:34:13 -0700 (PDT) Received: from [5.158.153.53] (helo=debian-buster-darwi.lab.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:RSA_AES_256_CBC_SHA1:256) (Exim 4.80) (envelope-from ) id 1jgUSu-0001eH-D8; Wed, 03 Jun 2020 16:34:00 +0200 Date: Wed, 3 Jun 2020 16:33:58 +0200 From: "Ahmed S. Darwish" To: Eric Dumazet Cc: Waiman Long , Peter Zijlstra , Ingo Molnar , Will Deacon , Thomas Gleixner , "Paul E. McKenney" , "Sebastian A. Siewior" , Steven Rostedt , LKML , "David S. Miller" , Jakub Kicinski , netdev@vger.kernel.org Subject: Re: [PATCH v1 01/25] net: core: device_rename: Use rwsem instead of a seqcount Message-ID: <20200603143358.GA1121436@debian-buster-darwi.lab.linutronix.de> References: <20200519214547.352050-1-a.darwish@linutronix.de> <20200519214547.352050-2-a.darwish@linutronix.de> <33cec6a9-2f6e-3d3c-99ac-9b2a3304ec26@gmail.com> <20200520064246.GA353513@debian-buster-darwi.lab.linutronix.de> <1498089b-08bd-d776-1570-d8b34463c702@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1498089b-08bd-d776-1570-d8b34463c702@gmail.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 20, 2020 at 05:51:27AM -0700, Eric Dumazet wrote: > > On 5/19/20 11:42 PM, Ahmed S. Darwish wrote: > > Hello Eric, > > > > On Tue, May 19, 2020 at 07:01:38PM -0700, Eric Dumazet wrote: > >> > >> On 5/19/20 2:45 PM, Ahmed S. Darwish wrote: > >>> Sequence counters write paths are critical sections that must never be > >>> preempted, and blocking, even for CONFIG_PREEMPTION=n, is not allowed. > >>> > >>> Commit 5dbe7c178d3f ("net: fix kernel deadlock with interface rename and > >>> netdev name retrieval.") handled a deadlock, observed with > >>> CONFIG_PREEMPTION=n, where the devnet_rename seqcount read side was > >>> infinitely spinning: it got scheduled after the seqcount write side > >>> blocked inside its own critical section. > >>> > >>> To fix that deadlock, among other issues, the commit added a > >>> cond_resched() inside the read side section. While this will get the > >>> non-preemptible kernel eventually unstuck, the seqcount reader is fully > >>> exhausting its slice just spinning -- until TIF_NEED_RESCHED is set. > >>> > >>> The fix is also still broken: if the seqcount reader belongs to a > >>> real-time scheduling policy, it can spin forever and the kernel will > >>> livelock. > >>> > >>> Disabling preemption over the seqcount write side critical section will > >>> not work: inside it are a number of GFP_KERNEL allocations and mutex > >>> locking through the drivers/base/ :: device_rename() call chain. > >>> > >>> From all the above, replace the seqcount with a rwsem. > >>> > >>> Fixes: 5dbe7c178d3f (net: fix kernel deadlock with interface rename and netdev name retrieval.) > >>> Fixes: 30e6c9fa93cf (net: devnet_rename_seq should be a seqcount) > >>> Fixes: c91f6df2db49 (sockopt: Change getsockopt() of SO_BINDTODEVICE to return an interface name) > >>> Cc: > >>> Signed-off-by: Ahmed S. Darwish > >>> Reviewed-by: Sebastian Andrzej Siewior > >>> --- > >>> net/core/dev.c | 30 ++++++++++++------------------ > >>> 1 file changed, 12 insertions(+), 18 deletions(-) > >>> > >> > >> Seems fine to me, assuming rwsem prevent starvation of the writer. > >> > > > > Thanks for the review. > > > > AFAIK, due to 5cfd92e12e13 ("locking/rwsem: Adaptive disabling of reader > > optimistic spinning"), using a rwsem shouldn't lead to writer starvation > > in the contended case. > > Hmm this was in linux-5.3, so very recent stuff. > > Has this patch been backported to stable releases ? > > With all the Fixes: tags you added, stable teams will backport this > networking patch to all stable versions. > > Do we have a way to tune a dedicare rwsem to 'give preference to the > (unique in this case) writer" over a myriad of potential readers ? > I was wrong in referencing the commit 5cfd92e12e13 above. Before and after that commit, once a rwsem writer is blocking, all subsequent readers will block until that writer makes progress. Given that behavior, and that the read section is already quite short, I don't think there's any danger incurred on writers here. (a v2 will be sent shortly, fixing the error found Dan/kbuild-bot.) Thanks, -- Ahmed S. Darwish Linutronix GmbH