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Date: Fri, 15 Jan 2016 09:21:25 +0100 (CET) From: Thomas Gleixner <tglx@...utronix.de> To: Hannes Frederic Sowa <hannes@...essinduktion.org> cc: Eric Dumazet <eric.dumazet@...il.com>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>, netdev@...r.kernel.org Subject: Re: [PATCH RT] net: move xmit_recursion to per-task variable on -RT 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
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