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    SubjectRe: [PATCH -rt 4/5] use migrate_disable for __local_begin
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    On Sat, 2007-07-14 at 14:35 -0400, Mathieu Desnoyers wrote:
    > * Peter Zijlstra (a.p.zijlstra@chello.nl) wrote:
    > > On Sat, 2007-07-14 at 13:16 -0400, Mathieu Desnoyers wrote:
    > > > * 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.
    > >
    > > Sure, but on -rt it does suffice, this part of the patch is rather WIP.
    > >
    > >
    >
    > Hrm, how can it suffice, I wonder ? migrate_disable() does not protect
    > against other threads on the same CPU, so you could suffer from
    > concurrent updates to the same variables. How is it different in -rt ?

    I thought the idea was that all these local_* operation were atomic wrt
    to the local cpu.

    The only difference with -rt is that we generally don't care about
    interrupts.

    Anyway, I'm dropping all this local stuff, and just hard code it right
    into slub.c



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