Messages in this thread | | | Date | Thu, 29 May 2008 22:17:55 -0700 (PDT) | From | Christoph Lameter <> | Subject | Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu operations |
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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
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