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

[Hannes Frederic Sowa]

On 15.01.2016 09:21, Thomas Gleixner wrote:
> 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.

Sorry for maybe writing it misleading but that is exactly what I wanted 
to say here. Yes, I agree, I didn't really check because of _bh and 
rcu_read_lock. This was a mistake. ;)

I already send out an updated patch with added preemption guards.

Thanks,
Hannes