Re: [tip:x86/mm] x86/pat: Avoid contention on cpa_lock if possible

[Peter Zijlstra]

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@xxxxxxxxxx>

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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