Re: FD array expansion problem

Matthew Wilcox (Matthew.Wilcox@genedata.com)
Sun, 31 Oct 1999 19:02:34 +0100

On Sun, Oct 31, 1999 at 01:08:42AM +0200, Ralf Baechle wrote:
> On Sat, Oct 30, 1999 at 03:42:17PM +0200, Matthew Wilcox wrote:
>
> > We don't have ll/sc instructions or compare-and-swap on pa-risc.
> > I'd really like to be able to do this without disabling interrupts,
> > but I don't think there's a way.
>
> Actually there is a neat way for uniprocessor MIPSes to implement this.
> The two registers k0 and k1 are reserved for use of the kernel, for example
> in interrupt processing. All exception handles will destroy the content
> of these registers and set it to a value != NULL which allows to implement
> an atomic increment for uniprocessors like:
>
> move k0, zero
> 1: lw old, addr
> sw new, addr
> bnez k0, 1b
>
> Dunno if that'd hold for HPPA as well.

I don't think that works. Look:

main code interrupt
set k0 to zero
load A from addr
store B at addr
set k0 to zero
load B from addr
store C at addr
return (i assume this makes k0 non-zero)
k0 is non-zero, so loop again
load C from addr
store B at addr

And you've lost the original A. I came up with a similar scheme while
taking a long walk today (many people seem to prefer the shower for their
good ideas. I find Japanese stroll gardens are ideal, and a nice long
walk is almost as good.)

The idea is to flag whether we're in the middle of an xchg operation in
a register such as k0, then have the interrupt handler's exit routine fix
it up:

move k0, one
lw A, addr
sw B. addr
move k0, zero

would then be the typical non-interrupted case.

The modified interrupt code looks like:

int xchging = (k0 == 1);

... normal irq code ...

if (!xchging)
return;
if (instruction we will return to is a store)
return to addr - 4;
return;

This should work, and have a minimal cost per interrupt.
Now to find out whether we have a suitable register on PA-RISC..

-- 
Matthew Wilcox <willy@bofh.ai>
"Windows and MacOS are products, contrived by engineers in the service of
specific companies. Unix, by contrast, is not so much a product as it is a
painstakingly compiled oral history of the hacker subculture." - N Stephenson


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