Re: [parisc-linux] Re: [PATCH] Fix the cpumask rewrite

[Jeff Garzik]

Linus Torvalds wrote:
> It _happens_ to be one on x86, so sadly it works on 99% of the machines 
> out there. And to avoid the extra (suprefluous) mb(), there are
>
> 	smp_mb__before_clear_bit()
> 	smp_mb__after_clear_bit()
>
> that only works with "clear_bit()", on the assumption that the way you'd 
> do locking is:
>
> 	lock:
> 		while (test_and_set_bit(..)) /* This is a memory barrier */
> 			while (test_bit(..))
> 				cpu_relax();
>
> 	.. protected region ..
>
> 	unlock:
> 		smp_mb__before_clear_bit();
> 		clear_bit(..);


FWIW one of the major uses of bitops currently is e.g. in 
include/linux/netdevice.h, where bitops are used for atomic selection of 
code paths, but not spinning:

static inline void netif_carrier_off(struct net_device *dev)
{
        if (!test_and_set_bit(__LINK_STATE_NOCARRIER, &dev->state))
                linkwatch_fire_event(dev);
}

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