On Wed, Jun 11, 2014 at 12:54:02PM +0200, Peter Zijlstra wrote:
> > @@ -252,6 +260,18 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> >  
> >  	BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
> >  
> > +#ifdef CONFIG_VIRT_UNFAIR_LOCKS
> > +	/*
> > +	 * A simple test and set unfair lock
> > +	 */
> > +	if (static_key_false(&virt_unfairlocks_enabled)) {
> > +		cpu_relax();	/* Relax after a failed lock attempt */
> 
> Meh, I don't think anybody can tell the difference if you put that in or
> not, therefore don't.
> 
> > +		while (!queue_spin_trylock(lock))
> > +			cpu_relax();
> > +		return;
> > +	}
> > +#endif /* CONFIG_VIRT_UNFAIR_LOCKS */
> 
> If you're really worried about those upper 24bits, you can always clear
> them when you get here.

I don't think its a problem at all; flipping the static_key requires
stop_machine, which guarantees us that there are no spinlocks held. So I
think you can actually BUG_ON() the upper 24bits.