Re: [PATCH] x86: modify inline asm constraints in __cmpxchg_double()
From: Juergen Gross
Date: Tue Oct 16 2018 - 03:43:40 EST
On 16/10/2018 08:25, Ingo Molnar wrote:
> * Juergen Gross <jgross@xxxxxxxx> 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@xxxxxxxx>
>> 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:
>> 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.