linux-kernel - Re: [gcv v3 06/35] scheduler: Replace __get_cpu

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Message-ID: <20130903114454.2a995f41@gandalf.local.home>
Date:	Tue, 3 Sep 2013 11:44:54 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Christoph Lameter <cl@...ux.com>, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>, akpm@...uxfoundation.org,
	Ingo Molnar <mingo@...hat.com>, linux-arch@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [gcv v3 06/35] scheduler: Replace __get_cpu_var uses

On Tue, 3 Sep 2013 16:45:45 +0200
Frederic Weisbecker <fweisbec@...il.com> wrote:

> 2013/9/3 Christoph Lameter <cl@...ux.com>:
> > On Thu, 29 Aug 2013, Steven Rostedt wrote:
> >
> >> How many places use the this_cpu_*() without preemption disabled? I
> >> wouldn't think there's many. I never complained about another variant,
> >> so you need to ask those that have. The tough question for me is what
> >> that variant name should be ;-)
> >
> > Tried to add preemption checks but the basic issue is that many of the
> > checks themselves use this_cpu_ops. percpu.h is very basic to the
> > operation of fundamental primitives for preempt etc. Use of a BUG_ON needs
> > a seris of includes in percpu.h that cause more trouble.
> >
> > If I switch __this_cpu ops to check for preemption then the logic for
> > preemption etc must use the raw_this_cpu ops.
> 
> IIUC the issue is that preempt debug checks themselves use per cpu
> operations that can result in preempt debug checks? Hence a recursion.
> Do you have an example of that?
> 
> Also in this case this must be fixed anyway given the checks that
> already exist in smp_processor_id(), __get_cpu_var(), ...

Right, that's why there's a raw_smp_processor_id() and
__raw_get_cpu_var(). Those two are the ones without checks, and they
are called by the non "raw" versions after the check is done.

Really, what's so damn hard about this?

-- Steve
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