Re: [PATCH -rt 4/5] use migrate_disable for __local_begin

Mathieu Desnoyers
2007-07-14T17:16:53Z

* Peter Zijlstra (a.p.zijlstra@chello.nl) wrote:
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
> include/asm-i386/local.h | 7 ++++---
> include/asm-x86_64/local.h | 7 ++++---
> 2 files changed, 8 insertions(+), 6 deletions(-)
>
> Index: linux-2.6/include/asm-i386/local.h
> ===================================================================
> --- linux-2.6.orig/include/asm-i386/local.h
> +++ linux-2.6/include/asm-i386/local.h
> @@ -197,11 +197,12 @@ static __inline__ long local_sub_return(
> #define __local_begin(__flags) \
> { \
> (__flags) = 0; \
> - preempt_disable(); \
> + migrate_disable(); \

Brrrr. That's wrong. Your non atomic __local*() updates only makes sense
when preempt_disable/enable() protects them from concurrent threads on
the same CPU, which is not the case of migrate_disable/enable(). This is
why I suggest that you use local_begin/end() mapped to
migrate_disable/enable() for normal local variables, and, if you really
want a __local_begin/end(), then it should be mapped to
preempt_disable/enable() and should state that it provides no protection
against interrupts.

Mathieu

> }
>
> -static inline void __local_end(unsigned long flags) {
> - preempt_enable();
> +static inline void __local_end(unsigned long flags)
> +{
> + migrate_enable();
> }
>
> /* On x86, these are no better than the atomic variants. */
> Index: linux-2.6/include/asm-x86_64/local.h
> ===================================================================
> --- linux-2.6.orig/include/asm-x86_64/local.h
> +++ linux-2.6/include/asm-x86_64/local.h
> @@ -186,11 +186,12 @@ static __inline__ long local_sub_return(
> #define __local_begin(__flags) \
> { \
> (__flags) = 0; \
> - preempt_disable(); \
> + migrate_disable(); \
> }
>
> -static inline void __local_end(unsigned long flags) {
> - preempt_enable();
> +static inline void __local_end(unsigned long flags)
> +{
> + migrate_enable();
> }
>
> #define __local_inc(l) local_inc(l)
>
> --
>

--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68