Re: [PATCH] x86: modify inline asm constraints in __cmpxchg_double()
From: Ingo Molnar
Date: Tue Oct 16 2018 - 02:26:03 EST
* 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?