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