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Subject: Re: [RFC][PATCH 0/5] Introduce checks for preemptable code for
 this_cpu_read/write()
From: Steven Rostedt <rostedt@goodmis.org>
To: Christoph Lameter <cl@gentwo.org>
Cc: Valdis.Kletnieks@vt.edu, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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Date: Tue, 20 Sep 2011 11:19:47 -0400
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On Tue, 2011-09-20 at 09:57 -0500, Christoph Lameter wrote:

> this_cpu_xx functions are made for those locations that have
> preemption enabled. If you can use those function (classic case is a
> per cpu counter increment in the network subsystem) then you can avoid
> preempt disable/enable or get_cpu/put_cpu.

If the variables are used for a very short time, then the latencies
introduced by a simple:

	var = get_cpu_var(my_var);
	if (var)
		do_something_quick();
	put_cpu_var(my_var);

Otherwise if that do_something_quick(); migrates, it may be doing
something it shouldn't be doing!

-- Steve