arch/powerpc: use BUILD_BUG() when detect unfit {cmp}xchg, size

From: pan xinhui <xinhui.pan@linux.vnet.ibm.com>

__xchg_called_with_bad_pointer() can't tell us what codes use {cmp}xchg
in incorrect way.  And no error will be reported until the link stage.
To fix such kinds of issues in a easy way, we use BUILD_BUG() here.

Signed-off-by: pan xinhui <xinhui.pan@linux.vnet.ibm.com>
---
 arch/powerpc/include/asm/cmpxchg.h | 19 +++++--------------
 1 file changed, 5 insertions(+), 14 deletions(-)

On Tue, Feb 23, 2016 at 04:45:16PM +0800, Pan Xinhui wrote:
> From: pan xinhui <xinhui.pan@linux.vnet.ibm.com>
> 
> __xchg_called_with_bad_pointer() can't tell us what codes use {cmp}xchg
> in incorrect way.  And no error will be reported until the link stage.
> To fix such kinds of issues in a easy way, we use BUILD_BUG() here.
> 
> Signed-off-by: pan xinhui <xinhui.pan@linux.vnet.ibm.com>
> ---
>  arch/powerpc/include/asm/cmpxchg.h | 19 +++++--------------
>  1 file changed, 5 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/cmpxchg.h b/arch/powerpc/include/asm/cmpxchg.h
> index d1a8d93..20c0a30 100644
> --- a/arch/powerpc/include/asm/cmpxchg.h
> +++ b/arch/powerpc/include/asm/cmpxchg.h
> @@ -5,6 +5,7 @@
>  #include <linux/compiler.h>
>  #include <asm/synch.h>
>  #include <asm/asm-compat.h>
> +#include <linux/bug.h>
>  
>  /*
>   * Atomic exchange
> @@ -92,12 +93,6 @@ __xchg_u64_local(volatile void *p, unsigned long val)
>  }
>  #endif
>  
> -/*
> - * This function doesn't exist, so you'll get a linker error
> - * if something tries to do an invalid xchg().
> - */
> -extern void __xchg_called_with_bad_pointer(void);
> -
>  static __always_inline unsigned long
>  __xchg(volatile void *ptr, unsigned long x, unsigned int size)
>  {
> @@ -109,7 +104,7 @@ __xchg(volatile void *ptr, unsigned long x, unsigned int size)
>  		return __xchg_u64(ptr, x);
>  #endif
>  	}
> -	__xchg_called_with_bad_pointer();
> +	BUILD_BUG();

Maybe we can use BUILD_BUG_ON_MSG(1, "Unsupported size for xchg"), which
could provide more information.

With or without this verbosity:

Acked-by: Boqun Feng <boqun.feng@gmail.com>


Regards,
Boqun

>  	return x;
>  }
>  
> @@ -124,7 +119,7 @@ __xchg_local(volatile void *ptr, unsigned long x, unsigned int size)
>  		return __xchg_u64_local(ptr, x);
>  #endif
>  	}
> -	__xchg_called_with_bad_pointer();
> +	BUILD_BUG();
>  	return x;
>  }
>  #define xchg(ptr,x)							     \
> @@ -235,10 +230,6 @@ __cmpxchg_u64_local(volatile unsigned long *p, unsigned long old,
>  }
>  #endif
>  
> -/* This function doesn't exist, so you'll get a linker error
> -   if something tries to do an invalid cmpxchg().  */
> -extern void __cmpxchg_called_with_bad_pointer(void);
> -
>  static __always_inline unsigned long
>  __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new,
>  	  unsigned int size)
> @@ -251,7 +242,7 @@ __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new,
>  		return __cmpxchg_u64(ptr, old, new);
>  #endif
>  	}
> -	__cmpxchg_called_with_bad_pointer();
> +	BUILD_BUG();
>  	return old;
>  }
>  
> @@ -267,7 +258,7 @@ __cmpxchg_local(volatile void *ptr, unsigned long old, unsigned long new,
>  		return __cmpxchg_u64_local(ptr, old, new);
>  #endif
>  	}
> -	__cmpxchg_called_with_bad_pointer();
> +	BUILD_BUG();
>  	return old;
>  }
>  
> -- 
> 2.5.0
>

On Tue, 2016-02-23 at 17:15 +0800, Boqun Feng wrote:
> On Tue, Feb 23, 2016 at 04:45:16PM +0800, Pan Xinhui wrote:
> > From: pan xinhui <xinhui.pan@linux.vnet.ibm.com>
> > 
> > diff --git a/arch/powerpc/include/asm/cmpxchg.h b/arch/powerpc/include/asm/cmpxchg.h
> > index d1a8d93..20c0a30 100644
> > --- a/arch/powerpc/include/asm/cmpxchg.h
> > +++ b/arch/powerpc/include/asm/cmpxchg.h
> > @@ -92,12 +93,6 @@ __xchg_u64_local(volatile void *p, unsigned long val)
> >  }
> >  #endif
> >  
> > -/*
> > - * This function doesn't exist, so you'll get a linker error
> > - * if something tries to do an invalid xchg().
> > - */
> > -extern void __xchg_called_with_bad_pointer(void);
> > -
> >  static __always_inline unsigned long
> >  __xchg(volatile void *ptr, unsigned long x, unsigned int size)
> >  {
> > @@ -109,7 +104,7 @@ __xchg(volatile void *ptr, unsigned long x, unsigned int size)
> >  		return __xchg_u64(ptr, x);
> >  #endif
> >  	}
> > -	__xchg_called_with_bad_pointer();
> > +	BUILD_BUG();
> 
> Maybe we can use BUILD_BUG_ON_MSG(1, "Unsupported size for xchg"), which
> could provide more information.

Yes I would prefer there was a message. Please send a v2.

cheers