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

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Date:   Tue, 16 Oct 2018 10:01:50 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Juergen Gross <jgross@...e.com>
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()


* Juergen Gross <jgross@...e.com> wrote:

> 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.

Ok. No sane workaround we can do to avoid the hang I suspect, right?

Should we propagate the fix/enhancement to x86/urgent? It's currently in 
x86/asm for a v4.20 merge.

Thanks,

	Ingo