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 -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
