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Subject: Re: [RFC][PATCH 0/5] Introduce checks for preemptable code for
 this_cpu_read/write()
From: Peter Zijlstra <peterz@infradead.org>
To: Christoph Lameter <cl@gentwo.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
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        Thomas Gleixner <tglx@linutronix.de>
Date: Tue, 20 Sep 2011 18:50:03 +0200
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On Tue, 2011-09-20 at 11:10 -0500, Christoph Lameter wrote:
> On Tue, 20 Sep 2011, Steven Rostedt wrote:
> 
> > I really mean all other users of this_cpu_*(), including the cmpxchg and
> > friends, still need to have preemption disabled.
> 
> This is argument against the basic design of this_cpu_ops. They were
> designed to avoid having to disable preemption for single operations on
> per cpu data. I think this shows a basic misunderstanding of what you are
> dealing with.

But part of that design is that its impossible to verify the
correctness. This is the part we object to and you keep avoiding.

There is a reason smp_processor_id() warns if its called in a
preemptible context, all the this_cpu wankery doesn't. It doesn't
provide a single useful debug feature and in places is designed so that
its impossible.

Seriously, how can you defend this shitpile with a straight face? Sure
it make slub go faster, but who gives a flying fuck if it brings the
rest of the kernel to its knees.