linux-kernel - Re: [tip:x86/mm] x86/pat: Avoid contention on cpa

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Date:	Wed, 06 Jun 2012 19:24:03 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	mingo@...nel.org, hpa@...or.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, shai@...lemp.com,
	akpm@...ux-foundation.org, tglx@...utronix.de, ido@...ery.com
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/mm] x86/pat: Avoid contention on cpa_lock if possible

On Wed, 2012-06-06 at 09:18 -0700, tip-bot for Shai Fultheim wrote:

> [ I absolutely hate these locking patterns ... yet I have no better idea. Maybe the gents on Cc: ... ]
> Signed-off-by: Ingo Molnar <mingo@...nel.org>

Oh yuck, this is vile..

static struct static_key scale_mp_trainwreck = STATIC_KEY_INIT_FALSE;

static DEFINE_SPINLOCK(_cpa_lock);

static inline void cpa_lock(void)
{
	if (static_key_false(&scale_mp_trainwreck))
		return;

	spin_lock(&_cpa_lock);
}

static inline void cpa_unlock(void)
{
	if (static_key_false(&scale_mp_trainwreck))
		return;

	spin_lock(&_cpa_lock);
}

And then use cpa_{,un}lock(), and the scale-mp guys can
static_key_slow_inc(&scale_mp_trainwreck).

[ and yes I hate those jump_label names ... but I'm not wanting
  to go through another round of bike-shed painting. ]
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