linux-kernel - Re: [PATCH] x86: modify inline asm constraints in __cmpxchg

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Date:   Tue, 16 Oct 2018 09:43:35 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, hpa@...or.com,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de
Subject: Re: [PATCH] x86: modify inline asm constraints in __cmpxchg_double()

On 16/10/2018 08:25, Ingo Molnar wrote:
> 
> * Juergen Gross <jgross@...e.com> wrote:
> 
>> Some gcc versions seem to have problems with the constraints in
>> __cmpxchg_double() as they suddenly issue a build error when random
>> parts of sources calling __cmpxchg_double() are modified, like e.g.
>> slub.c. This has been observed on Debian systems only so far.
>>
>> Using "0" instead of "a" in the input constraints has the same
>> semantics while avoiding that build error.
>>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
>> ---
>> I should note that I have observed gcc hangs instead sometimes. Not
>> taking any patches modifying users of __cmpxchg_double() due to a gcc
>> bug which seems to be distro-specific is a bad move IMO.
>>
>> I'd rather make it clear from build behavior that this is a bug in gcc
>> by letting the build hang instead of throwing error warnings not in any
>> way related to changes in the code.
>>
>> The gcc bug is filed under:
>>
>> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=908154
>> ---
>>  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));			\
> 
> This got changed to +a in -tip:
> 
>   c808c09b527c: x86/asm: Use CC_SET()/CC_OUT() in __cmpxchg_double()
> 
> Mind sending a patch on top of -tip?

No, I think you can drop my patch.

c808c09b527c: x86/asm: Use CC_SET()/CC_OUT() in __cmpxchg_double()
has the same effect as my patch: the build failure is replaced by
gcc hanging instead in the critical configuration.


Juergen