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

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Message-ID: <alpine.DEB.2.00.1109201108080.8056@router.home>
Date:	Tue, 20 Sep 2011 11:08:56 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Valdis.Kletnieks@...edu, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [RFC][PATCH 0/5] Introduce checks for preemptable code for
 this_cpu_read/write()

On Tue, 20 Sep 2011, Steven Rostedt wrote:

> 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!

That is obviously a use case in which this_cpu_xx ops could not be used
since we must stay on the same cpu. I think there is still a lot of
confusion on your part. The example here shows it.

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