From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756195AbcAOIW2 (ORCPT ); Fri, 15 Jan 2016 03:22:28 -0500 Received: from www.linutronix.de ([62.245.132.108]:33489 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754464AbcAOIW1 (ORCPT ); Fri, 15 Jan 2016 03:22:27 -0500 Date: Fri, 15 Jan 2016 09:21:25 +0100 (CET) From: Thomas Gleixner To: Hannes Frederic Sowa cc: Eric Dumazet , Sebastian Andrzej Siewior , linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, Steven Rostedt , netdev@vger.kernel.org Subject: Re: [PATCH RT] net: move xmit_recursion to per-task variable on -RT In-Reply-To: <56982895.6070901@stressinduktion.org> Message-ID: References: <20160113152352.GH29964@linutronix.de> <20160114145007.GC17776@linutronix.de> <56981AE1.20202@stressinduktion.org> <1452810004.1223.154.camel@edumazet-glaptop2.roam.corp.google.com> <56982895.6070901@stressinduktion.org> User-Agent: Alpine 2.11 (DEB 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII 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 List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 15 Jan 2016, Hannes Frederic Sowa wrote: > On 14.01.2016 23:20, Eric Dumazet wrote: > > On Thu, 2016-01-14 at 23:02 +0100, Hannes Frederic Sowa wrote: > > > > > We are just adding a second recursion limit solely to openvswitch which > > > has the same problem: > > > > > > https://patchwork.ozlabs.org/patch/566769/ > > > > > > This time also we depend on rcu_read_lock marking the section being > > > nonpreemptible. Nice would be a more generic solution here which doesn't > > > need to always add something to *current. > > > > > > Note that rcu_read_lock() does not imply that preemption is disabled. > > Exactly, it is conditional on CONFIG_PREEMPT_CPU/CONFIG_PREMPT_COUNT but > haven't thought about exactly that in this moment. Wrong. CONFIG_PREEMPT_RCU makes RCU preemptible. If that is not set then it fiddles with preempt_count when CONFIG_PREEMPT_COUNT=y. If CONFIG_PREEMPT_COUNT=n then you have a non preemptible system anyway. So you cannot assume that rcu_read_lock() disables preemption. Thanks, tglx