Messages in this thread | | | Date | Thu, 29 May 2008 23:12:59 -0700 (PDT) | From | Christoph Lameter <> | Subject | Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu operations |
| |
On Thu, 29 May 2008, Andrew Morton wrote:
> > area. It requies disabling of interrupts etc. > > No it doesn't. Look: > > static inline void local_inc(local_t *l) > { > asm volatile(_ASM_INC "%0" > : "+m" (l->a.counter)); > } > > > Its not atomic (wrt > > interrupts) because of that. > > > > Yes it is.
No its not! In order to increment a per cpu value you need to calculate the per cpu pointer address in the current per cpu segment. local_t cannot do that in an atomic (wrt interrupt/preempt fashion) fashion. cpu ops can use a segment prefix and thus the insructions can calculate the per cpu adress and perform the atomic inc without disabling preempt or interrupts.
local_t is only useful when you disable interrupt or premption otherwise. But then you could also use a regular increment.
> > 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 > > Well that should be fixed. We should never have mixed the > alloc_percpu() and DEFINE_PER_CPU things inthe same header. They're > different.
With cpu_alloc they are the same. They allocate from the same per cpu area.
| |