linux-kernel - Re: [PATCH RT] net: move xmit_recursion to per-task variable on -RT

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Message-ID: <alpine.DEB.2.11.1601150916180.3575@nanos>
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