linux-kernel - Re: x86: A fast way to check capabilities of the current cpu

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Message-ID: <20110121171133.GI2832@htj.dyndns.org>
Date:	Fri, 21 Jan 2011 18:11:33 +0100
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Pekka Enbeerg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	akpm@...ux-foundation.org
Subject: Re: x86: A fast way to check capabilities of the current cpu

Hello, Christoph.

I was trying to forward this to x86 tree but spotted a problem.

On Wed, Dec 15, 2010 at 02:07:39PM -0600, Christoph Lameter wrote:
> +static __always_inline int this_cpu_constant_test_bit(unsigned int nr,
> +                        const unsigned long __percpu *addr)
> +{
> +	unsigned long __percpu *a = (unsigned long *)addr + nr / BITS_PER_LONG;
> +
> +	return ((1UL << (nr % BITS_PER_LONG)) & percpu_read_stable(*a)) != 0;
> +}

I don't think percpu_read_stable() can be used here.  It's not
guaranteed to be stable across different cpus.

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.

Thank you.

-- 
tejun
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