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Date:   Thu, 6 Sep 2018 15:54:06 +0200
From:   Juergen Gross <>
Subject: Re: cmpxchg.h:245:2: error: ‘asm’ operand has impossible constraints

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...


View attachment "0001-x86-correct-inline-asm-constraints-in-__cmpxchg_doub.patch" of type "text/x-patch" (1149 bytes)

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