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

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Date:	Tue, 3 Sep 2013 16:45:45 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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

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(), ...
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