Re: Vmstat: Small revisions to refresh_cpu_vm_stats()

[David Miller <davem@davemloft.net>]

> : undefined reference to `__xchg_called_with_bad_pointer'
> 
> This is sparc64's way of telling you that you can'd do xchg on an s8.
> 
> Dave, is that fixable?
> 
> I assume not, in which case we either go for some open-coded implementation
> for 8- and 16-bits or we should ban (at compile time) 8- and 16-bit xchg on
> all architectures.

Right, let's write some generic code for this because other platforms
are going to need this too.

Basically, do a normal "ll/sc" or "load/cas" sequence on a u32 with
some shifting and masking as needed.

	int shift = (((unsigned long) addr) % 4) * 8;
	unsigned long mask = 0xff << shift;
	unsigned long val = newval << shift;
	u32 *ptr = (u32 *) ((unsigned long)addr & ~0x3UL);

	while (1) {
		u32 orig, tmp = *ptr;

		orig = tmp;
		tmp &= ~mask;
		tmp |= val;
		cmpxchg_u32(ptr, orig, tmp);
		if (orig == tmp)
			break;
	}

Repeat for u16, etc.

However, for platforms like sparc32 that can do a xchg() atomically
but can't do cmpxchg, this idea won't work :-/

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