Re: [PATCH] asm-generic: fix compilation failure in cmpxchg_double()

[Arnd Bergmann]

On Thu, Mar 23, 2017 at 9:49 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Wed, Mar 22, 2017 at 10:27 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>> On Wed, Mar 22, 2017 at 12:27 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>>> On Wed, Mar 22, 2017 at 12:10 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>>>> Arnd reported that the new code leads to compilation failures
>>>> with some versions of gcc. I've filed gcc issue 72873,
>>>> but we need a kernel fix as well.
>>>>
>>>> Remove instrumentation from cmpxchg_double() for now.
>>>
>>> Thanks, I also checked that fixes the build error for me.
>>
>> I got a new variant of the bug in
>> arch/x86/include/asm/cmpxchg_32.h:set_64bit() now.
>>
>> In file included from /git/arm-soc/arch/x86/include/asm/cmpxchg.h:142:0,
>>                  from /git/arm-soc/arch/x86/include/asm/atomic.h:7,
>>                  from /git/arm-soc/arch/x86/include/asm/msr.h:66,
>>                  from /git/arm-soc/arch/x86/include/asm/processor.h:20,
>>                  from /git/arm-soc/arch/x86/include/asm/cpufeature.h:4,
>>                  from /git/arm-soc/arch/x86/include/asm/thread_info.h:52,
>>                  from /git/arm-soc/include/linux/thread_info.h:25,
>>                  from /git/arm-soc/arch/x86/include/asm/preempt.h:6,
>>                  from /git/arm-soc/include/linux/preempt.h:80,
>>                  from /git/arm-soc/include/linux/spinlock.h:50,
>>                  from /git/arm-soc/include/linux/mmzone.h:7,
>>                  from /git/arm-soc/include/linux/gfp.h:5,
>>                  from /git/arm-soc/include/linux/mm.h:9,
>>                  from /git/arm-soc/mm/khugepaged.c:3:
>> /git/arm-soc/mm/khugepaged.c: In function 'khugepaged':
>> /git/arm-soc/arch/x86/include/asm/cmpxchg_32.h:29:2: error: 'asm'
>> operand has impossible constraints
>>   asm volatile("\n1:\t"
>>
>> Defconfig is at http://pastebin.com/raw/Pthhv5iU
>
>
> I can't reproduce it with gcc 4.8.4, 7.0.0, 7.0.1.
>
> Are you sure it's related to my recent change? I did not touch set_64bit.

You are right, this is different, it just appeared on the same day with
almost exactly the same symptom as the other one, so I mistakenly
assumed it was the same root cause.

Reverting your patches doesn't fix it, and I only see it with the
latest gcc-7.0.1 snapshot, not with one from a few weeks ago.
I'll open a gcc bug for it.

       Arnd