Re: [PATCH v2] LLVMLinux: Remove warning about returning an uninitialized variable

[Behan Webster]

On 03/22/14 09:42, James Bottomley wrote:
> On Sat, 2014-03-22 at 09:37 -0700, Behan Webster wrote:
> > On 03/22/14 09:29, James Bottomley wrote:
> > > On Sat, 2014-03-22 at 08:48 -0700, behanw@xxxxxxxxxxxxxxxxxx wrote:
> > > > From: Behan Webster <behanw@xxxxxxxxxxxxxxxxxx>
> > > >
> > > > Fix uninitialized return code in default case in cmpxchg-local.h
> > > >
> > > > This patch fixes the code to prevent an uninitialized return value that is detected
> > > > when compiling with clang. The bug produces numerous warnings when compiling the
> > > > Linux kernel with clang.
> > > >
> > > > Signed-off-by: Behan Webster <behanw@xxxxxxxxxxxxxxxxxx>
> > > > Signed-off-by: Mark Charlebois <charlebm@xxxxxxxxx>
> > > > ---
> > > >    include/asm-generic/cmpxchg-local.h | 1 +
> > > >    1 file changed, 1 insertion(+)
> > > >
> > > > diff --git a/include/asm-generic/cmpxchg-local.h b/include/asm-generic/cmpxchg-local.h
> > > > index d8d4c89..9112111 100644
> > > > --- a/include/asm-generic/cmpxchg-local.h
> > > > +++ b/include/asm-generic/cmpxchg-local.h
> > > > @@ -41,6 +41,7 @@ static inline unsigned long __cmpxchg_local_generic(volatile void *ptr,
> > > >    		break;
> > > >    	default:
> > > >    		wrong_size_cmpxchg(ptr);
> > > > +		__builtin_unreachable();
> > > No, that's got to be unreachable() so that it works in all compilers,
> > > (__builtin_unreachable is a gcc-4 ism).
> > It is also supported by clang.
> OK, but it's not supported by gcc-3.  So gcc-3 would crap out seeing the
> statement, which is why we have to wrapper it.
Sorry. I wasn't clear. It is not merely a gcc-4 ism; it is also 
supported by clang.

I'm not arguing it should be used. I understand (and agree with) your 
objection to its use since it isn't supported before gcc-4.

> > > Got to say this still looks wrong.  If wrong_size_cmpxchg() cannot
> > > return, the function should be annotated as such with __noreturn (like
> > > panic()) so the unreachable() should be superfluous.
> > Okay. I can try that instead.
> Great; This seems to work for me (but then my compiler doesn't see the
> unreachable problem).
>
> James
>
> ---
> diff --git a/include/asm-generic/cmpxchg-local.h b/include/asm-generic/cmpxchg-local.h
> index d8d4c89..70bef78 100644
> --- a/include/asm-generic/cmpxchg-local.h
> +++ b/include/asm-generic/cmpxchg-local.h
> @@ -4,7 +4,8 @@
>   #include <linux/types.h>
>   #include <linux/irqflags.h>
>   
> -extern unsigned long wrong_size_cmpxchg(volatile void *ptr);
> +extern unsigned long wrong_size_cmpxchg(volatile void *ptr)
> +	__noreturn;
>   
>   /*
>    * Generic version of __cmpxchg_local (disables interrupts). Takes an unsigned
Nice. I'll give it a try. It looks like a much better solution.

Behan

--
Behan Webster
behanw@xxxxxxxxxxxxxxxxxx

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