linux-kernel - Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical section?

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Date:	Mon, 9 Sep 2013 15:24:55 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, darren@...art.com,
	sbw@....edu
Subject: Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical
 section?

On Mon, 9 Sep 2013, Peter Zijlstra wrote:

> > Slander. Certainly validation is good. Its just that PREEMPT kernels are
> > not in use
>
> Complete bullshit, its part of the mainline kernel, lots of people run
> them -- including me, and any patch is supposed to keep it working.

Nonsense. There is no main line distro that supports full preempt. Its an
academic exercise.

> > and AFAICT the full preempt stuff requires significant developer
> > support and complicates the code without much benefit.
>
> More bullshit, each and every patch submitted must fully support all
> preemption modes supported by the kernel. CONFIG_PREEMPT is in, no two
> ways about it. Breaking it is a regression and reason to revert stuff.

Right so you have enforcing that developers spend time to maintain a
useless kernel option. We have lots of other things to worry about.

> Therefore every Linux developer supports it per definition. And clearly
> the complication is worth it for enough people, otherwise it wouldn't be
> there.

Where is the worth? The only thing that I heard last time that I
brought it up is that there is some audio specialty distro. Guess that
needs it to support broken old audio boards? Cannot believe that a kernel
with PREEMPT_VOLUNTARY cannot handle audio. All my Linux workstations do
just fine without full preemption.

It seems that even RT is moving away from full preemption due to the
performance issue. Wake up!!

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