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Date: Fri, 21 Jan 2011 11:46:04 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
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To: Tejun Heo <tj@kernel.org>
cc: "H. Peter Anvin" <hpa@zytor.com>, Pekka Enbeerg <penberg@cs.helsinki.fi>,
        linux-kernel@vger.kernel.org, Eric Dumazet <eric.dumazet@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        akpm@linux-foundation.org
Subject: Re: x86: A fast way to check capabilities of the current cpu
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On Fri, 21 Jan 2011, Tejun Heo wrote:

> On Fri, Jan 21, 2011 at 11:21:02AM -0600, Christoph Lameter wrote:
> > > I don't think percpu_read_stable() can be used here.  It's not
> > > guaranteed to be stable across different cpus.
> >
> > Why would that matter? The caller has to disabled preemption anyways since
> > otherwise the processor may change which means that the result of the
> > operation is useless.
>
> Because
>
> 	preempt_disable();
> 	this_cpu_has();
> 	preempt_enable();
> 	preempt_disable();
> 	this_cpu_has();
> 	preempt_enable();
>
> might malfunction.  percpu_read_stable() is pretty much applicable
> only to stuff local to the thread.

Ok then lets change it to percpu_read

> > > Also, can we just implement what's necessary on top of this_cpu_has()?
> > > this_cpu_has() already has constant handling, so there's no need to
> > > add this_cpu_test_bit() at this point.
> >
> > Not sure what you mean. this_cpu_test_bit is necessary because
> > test_cpu_cap expects a regular pointer and performs a regular load.
> > this_cpu_constant_test_bit handles the segment prefix necessary for a per
> > cpu load.
> >
> > The constant refers to the bit.
>
> Oh, you're right.  Sorry about that.  Can you please then add a
> comment noting that the operation is x86 only?  Maybe prefix it with
> x86_?

For a function defined in an specific include file and only used in arch
specific code?