network statistics in SMP

[Hans Petter Selasky]

On Saturday 19 December 2009 12:27:12 Ulrich Spörlein wrote:
> On Tue, 15.12.2009 at 13:13:28 -0500, John Baldwin wrote:
> > On Tuesday 15 December 2009 12:45:13 pm Harti Brandt wrote:
> > > I see. I was also thinking along these lines, but was not sure whether
> > > it is worth the trouble. I suppose this does not help to implement
> > > 64-bit counters on 32-bit architectures, though, because you cannot
> > > read them reliably without locking to sum them up, right?
> >
> > Either that or you just accept that you have a small race since it is
> > only stats. :)
>
> This might be stupid, but can we not easily *read* 64bit counters
> on 32bit machines like this:
>
> do {
>     h1 = read_upper_32bits;
>     l1 = read_lower_32bits;
>     h2 = read_upper_32bits;
>     l2 = read_lower_32bits; /* not needed */
> } while (h1 != h2);


Hi,

Just a comment. You know you don't need a while loop to get a stable value? 
Should be implemented like this, in my opinion:

h1 = read_upper_32bits;
l1 = read_lower_32bits;
h2 = read_upper_32bits;

if (h1 != h2)
	l1 = 0xffffffffUL;

sum64 = (h1<<32) | l1;

--HPS