Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu operations

Christoph Lameter
2008-05-30T05:18:11Z

On Thu, 29 May 2008, Andrew Morton wrote:

> > local_irq_save(flags);
> > /* Calculate address of per processor area */
> > p = CPU_PTR(stat, smp_processor_id());
> > p->counter++;
> > local_irq_restore(flags);
>
> eh? That's what local_t is for?

No that is what local_t exactly cannot do.

> > The segment can be replaced by a single atomic CPU operation:
> >
> > CPU_INC(stat->counter);
>
> hm, I guess this _has_ to be implemented as a macro. ho hum. But
> please: "cpu_inc"?

A lowercase macro?

> > The existing methods in use in the kernel cannot utilize the power of
> > these atomic instructions. local_t is not really addressing the issue
> > since the offset calculation performed before the atomic operation. The
> > operation is therefor not atomic. Disabling interrupt or preemption is
> > required in order to use local_t.
>
> Your terminology is totally confusing here.
>
> To me, an "atomic operation" is one which is atomic wrt other CPUs:
> atomic_t, for example.
>
> Here we're talking about atomic-wrt-this-cpu-only, yes?

Right.

> > local_t is also very specific to the x86 processor.
>
> And alpha, m32r, mips and powerpc, methinks. Probably others, but
> people just haven't got around to it.

No local_t does not do the relocation of the address to the correct percpu
area. It requies disabling of interrupts etc. Its not atomic (wrt
interrupts) because of that.

> I think I'll need to come back another time to understand all that ;)
>
> Thanks for writing it up carefully.

Well this stuff is so large in scope that I have difficulties keeping
everything straight.

> I wonder if all this stuff should be in a new header file.
>
> We could get lazy and include that header from percpu.h if needed.

But then its related to percpu operations and relies extensively on the
various percpu.h files in asm-generic and asm-arch and include/linux