From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Gleixner Subject: Re: [PATCH RT] net: move xmit_recursion to per-task variable on -RT Date: Fri, 15 Jan 2016 09:21:25 +0100 (CET) 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> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Cc: Eric Dumazet , Sebastian Andrzej Siewior , linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, Steven Rostedt , netdev@vger.kernel.org To: Hannes Frederic Sowa Return-path: In-Reply-To: <56982895.6070901@stressinduktion.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.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