Re: Calling current() from interrupt context

From: Kenn Humborg (
Date: Mon Oct 09 2000 - 17:45:18 EST

On Tue, Oct 10, 2000 at 12:36:35AM +0200, Andi Kleen wrote:
> On Mon, Oct 09, 2000 at 11:30:50PM +0100, Alan Cox wrote:
> > > I think I'll go for the 'current is in a well-known register'
> > > approach and see how this goes...
> >
> > Failing that the 2.0 approach will work, current is a global in uniprocessor
> > and a #define to an array indexed by cpu id in smp
> The problem is where to get the cpuid from (see how smp_processor_id
> is currently defined ;) When you don't have a hidden register in the
> CPU you're screwed.
> [x86-64 has one btw]

Simple. Each interrupt stack is, say, 8 pages. You have an array
of N interrupt stacks. Then you calculate

   cpu_id = (sp & ~(INT_STACK_SIZE-1)) >> (PAGE_SHIFT + 3);

Actually, I'd put the interrupt stack and any other per-cpu data
structures together in this region.

I don't know yet how you decide which secondary processor is which
at boot time. Maybe it doesn't matter, so you can just let them
fight over the per-cpu data structures by trying to claim spinlocks
on each one in turn.

Anyway, this SMP stuff will be quite academic for a while unless
someone wants to donate a workstation-sized SMP VAX (if such a
beast exists at all :-)


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