Re: Vmstat: Small revisions to refresh_cpu_vm_stats()

David Miller
2007-11-13T11:47:37Z

> : 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 :-/