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

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Message-ID: <5698BD29.8090405@stressinduktion.org>
Date:	Fri, 15 Jan 2016 10:34:33 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Thomas Gleixner <tglx@...utronix.de>
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 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