Re: Vmstat: Small revisions to refresh_cpu_vm_stats()

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 13 Nov 2007 03:47:37 -0800 (PST) David Miller <davem@davemloft.net> wrote:

> From: Andrew Morton <akpm@linux-foundation.org>
> Date: Tue, 13 Nov 2007 03:37:55 -0800
> 
> > : 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.

ok.  I guess if x86 can do it in hardware then it's worthwhile.

> 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 :-/

xchg() is nonatomic wrt other CPUs, so I think we can get by with
local_irq_save()/swap()/local_irq_restore().

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