Re: cmpxchg.h:245:2: error: ‘asm’ operand has impossible constraints

[Juergen Gross <jgross@suse.com>]

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Hey Meelis,

On Mon, 27 Aug 2018, Meelis Roos wrote:

> While trying to compile v4.18-13105-gaba16dc5cf93 with gcc 5.3.1 on a
> 32-bit x86 configured for AMD K6
>   CC      mm/slub.o
> In file included from ./arch/x86/include/asm/atomic.h:8:0,
>                  from ./include/linux/atomic.h:7,
>                  from ./arch/x86/include/asm/thread_info.h:54,
>                  from ./include/linux/thread_info.h:38,
>                  from ./arch/x86/include/asm/preempt.h:7,
>                  from ./include/linux/preempt.h:81,
>                  from ./include/linux/spinlock.h:51,
>                  from ./include/linux/mmzone.h:8,
>                  from ./include/linux/gfp.h:6,
>                  from ./include/linux/mm.h:10,
>                  from mm/slub.c:13:
> mm/slub.c: In function ‘__slab_free’:
> ./arch/x86/include/asm/cmpxchg.h:245:2: error: ‘asm’ operand has
impossible constraints
>   asm volatile(pfx "cmpxchg%c4b %2; sete %0"   \

Could you give the attched patch a try? I've been hit by this issue
via kbuild robot complaining about one of my patches which didn't
touch the asm stuff above.

I suspect the "=a" contraint of the outputs and "a" of the inputs
are the culprit. You could save me from having to search through
all the debian repos for a gcc showing that issue...


Juergen

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From 252450d28ffc8fbfd5b332d0b97990ae12a207d9 Mon Sep 17 00:00:00 2001
From: Juergen Gross <jgross@suse.com>
Date: Wed, 5 Sep 2018 11:32:26 +0200
Subject: [PATCH] x86: correct inline asm constraints in __cmpxchg_double()

Specifying "=a" for output constraints and "a" for input seems to be
wrong. Instead "0" should be used for the input constraint.

Fixes: cdcd629869fab ("x86: Fix and improve cmpxchg_double{,_local}()")
Cc: stable@vger.kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>
---
 arch/x86/include/asm/cmpxchg.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h
index a55d79b233d3..b3b4d61a8969 100644
--- a/arch/x86/include/asm/cmpxchg.h
+++ b/arch/x86/include/asm/cmpxchg.h
@@ -245,7 +245,7 @@ extern void __add_wrong_size(void)
 	asm volatile(pfx "cmpxchg%c4b %2; sete %0"			\
 		     : "=a" (__ret), "+d" (__old2),			\
 		       "+m" (*(p1)), "+m" (*(p2))			\
-		     : "i" (2 * sizeof(long)), "a" (__old1),		\
+		     : "i" (2 * sizeof(long)), "0" (__old1),		\
 		       "b" (__new1), "c" (__new2));			\
 	__ret;								\
 })
-- 
2.16.4