linux-kernel - Re: [RFC][PATCH 0/5] Introduce checks for preemptable code for this_cpu

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Date:	Mon, 19 Sep 2011 16:49:47 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC][PATCH 0/5] Introduce checks for preemptable code for
 this_cpu_read/write()

On Mon, 19 Sep 2011, Steven Rostedt wrote:

> I just found out that the this_cpu_*() functions do not perform the
> test to see if the usage is in atomic or not. Thus, the blind
> conversion of the per_cpu(*, smp_processor_id()) and the get_cpu_var()
> code to this_cpu_*() introduce the regression to detect the hard
> to find case where a per cpu variable is used in preempt code that
> migrates and causes bugs.

this_cpu_* function can be used either way. There is no point in checking
for atomic or not since the this_cpu_* implementations are required to
take care of disabling preemption. Those operations are generally safe to
use regardless of the context.

It can be replaced by __this_cpu_* only when we know that the context
prevents races through disabling preemption.
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