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

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Date:	Sat, 22 Mar 2014 23:32:21 -0700
From:	Behan Webster <behanw@...verseincode.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
CC:	arnd@...db.de, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, dwmw2@...radead.org,
	pageexec@...email.hu, Mark Charlebois <charlebm@...il.com>
Subject: Re: [PATCH v2] LLVMLinux: Remove warning about returning an uninitialized
 variable

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@...verseincode.com wrote:
>>>> From: Behan Webster <behanw@...verseincode.com>
>>>>
>>>> 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@...verseincode.com>
>>>> Signed-off-by: Mark Charlebois <charlebm@...il.com>
>>>> ---
>>>>    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@...verseincode.com

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