Re: linux-next: manual merge of the xen-tip tree with the tip tree

Andy Lutomirski
2015-08-12T18:26:31Z

On Wed, Aug 12, 2015 at 11:17 AM, Boris Ostrovsky
<boris.ostrovsky@oracle.com> wrote:
> On 08/12/2015 01:46 PM, Peter Zijlstra wrote:
>>
>> On Wed, Aug 12, 2015 at 07:21:05PM +0200, Peter Zijlstra wrote:
>>>
>>> On Wed, Aug 12, 2015 at 09:27:38AM -0400, Boris Ostrovsky wrote:
>>>
>>>> Incidentally, 11276d53 ("locking/static_keys: Add a new static_key
>>>> interface") breaks old-ish compilers (gcc version 4.4.4 20100503 (Red
>>>> Hat
>>>> 4.4.4-2) (GCC)):
>>>>
>>>>
>>>>
>>>> CC arch/x86/kernel/nmi.o
>>>> In file included from
>>>> /home/build/linux-boris/include/linux/jump_label.h:109,
>>>> from
>>>> /home/build/linux-boris/arch/x86/include/asm/spinlock.h:5,
>>>> from
>>>> /home/build/linux-boris/include/linux/spinlock.h:88,
>>>> from /home/build/linux-boris/arch/x86/kernel/nmi.c:14:
>>>> /home/build/linux-boris/arch/x86/include/asm/jump_label.h: In function
>>>> ‘nmi_handle’:
>>>> /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21: warning:
>>>> asm
>>>> operand 0 probably doesn’t match constraints
>>>> /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21: error:
>>>> impossible constraint in ‘asm’
>>>> make[3]: *** [arch/x86/kernel/nmi.o] Error 1
>>>> make[2]: *** [arch/x86/kernel] Error 2
>>>> make[1]: *** [arch/x86] Error 2
>>>
>>> Ugh bugger.
>>>
>>> I bet its that: &((char *)key)[branch] business, an earlier variant
>>> thereof tripped up more recent GCCs too.
>>>
>>> So its an __always_inline function, and both argument are always compile
>>> time constants, @key is the address of an object in static storage (a
>>> global) and @branch is a simple 0/1 at the call site.
>>>
>>> Now we wish to compute (unsigned long)key + branch at compile/link time
>>> to feed to the assembler as an immediate, which should be possible,
>>> given its all 'constants'.
>>>
>>> It just appears GCC is having a hard time with this.
>>>
>>> Let me see if I have a sufficiently old GCC around to play with.
>>
>> Could you feed the below to your compiler? Its a bit cumbersome, but
>> its the next best I could come up with...
>
>
> No, it produces the same error. This is Fedora 13, btw, uses gcc 4.4.4.

Is the problem just that it's being misdetected as supporting asm
goto? What does gcc -E say?

--Andy