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

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Date:	Sat, 22 Mar 2014 09:42:38 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Behan Webster <behanw@...verseincode.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 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.

> > 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


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