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

["H. Peter Anvin" <hpa@zytor.com>]

On 06/06/2012 10:24 AM, Peter Zijlstra wrote:
> 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@kernel.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).
> 

Actually, for this particular subcase I would use a synthetic CPUID bit
and use static_cpu_has().

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.