Re: [PATCH] unbreak alpha percpu

Martin Schwidefsky
2009-04-14T07:46:20Z

On Fri, 10 Apr 2009 09:21:08 -0700 (PDT)
Linus Torvalds <torvalds@linux-foundation.org> wrote:

>
> Hmm. I certainly personally _much_ prefer this version.
>
> It looks like this actually simplifies things for S390 too (no more
> simple_identifier_##var games), and generally just looks better. And
> seeing that the S390 special case is no longer S390-specific is also a
> good sign, imho.
>
> Sorry for including the whole message, but I'm adding s390 and percpu
> people to the cc. Guys - original uncorrupted patch on lkml.
> > diff --git a/arch/s390/include/asm/percpu.h b/arch/s390/include/asm/percpu.h
> > index 408d60b..6a71d73 100644
> > --- a/arch/s390/include/asm/percpu.h
> > +++ b/arch/s390/include/asm/percpu.h
> > @@ -13,20 +13,18 @@
> > */
> > #if defined(__s390x__) && defined(MODULE)
> >
> > -#define SHIFT_PERCPU_PTR(ptr,offset) (({ \
> > - extern int simple_identifier_##var(void); \
> > +#define SHIFT_PERCPU_PTR(var,offset) (({ \
> > unsigned long *__ptr; \
> > asm ( "larl %0, %1@GOTENT" \
> > - : "=a" (__ptr) : "X" (ptr) ); \
> > - (typeof(ptr))((*__ptr) + (offset)); }))
> > + : "=a" (__ptr) : "X" (&per_cpu_var(var)) ); \
> > + (typeof(&per_cpu_var(var)))((*__ptr) + (offset)); }))
> >
> > #else
> >
> > -#define SHIFT_PERCPU_PTR(ptr, offset) (({ \
> > - extern int simple_identifier_##var(void); \
> > +#define SHIFT_PERCPU_PTR(var, offset) (({ \
> > unsigned long __ptr; \
> > - asm ( "" : "=a" (__ptr) : "0" (ptr) ); \
> > - (typeof(ptr)) (__ptr + (offset)); }))
> > + asm ( "" : "=a" (__ptr) : "0" (&per_cpu_var(var)) ); \
> > + (typeof(&per_cpu_var(var))) (__ptr + (offset)); }))
> >
> > #endif
> >

I would like to get rid of that SHIFT_PERCPU_PTR mess. The patch from
Ivan will allow this, it uses a dummy variable to void the effect of a
static modifier for percpu variables in modules. The percpu variable
itself will be defined non-static, my gut feeling is that this is a
dirty little trick that might bite us in the future.
Another solution which I personally would prefer is to ban the use of
static percpu variables. Then the compiler will use the GOT to get the
address of percpu variables without any dirty tricks.

--
blue skies,
Martin.

"Reality continues to ruin my life." - Calvin.