On 16/10/2018 10:01, Ingo Molnar wrote: > > * Juergen Gross <jgr...@suse.com> wrote: > >> On 16/10/2018 08:25, Ingo Molnar wrote: >>> >>> * Juergen Gross <jgr...@suse.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 <jgr...@suse.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?
I'm not aware of any. > Should we propagate the fix/enhancement to x86/urgent? It's currently in > x86/asm for a v4.20 merge. My patch triggering the issue is queued for 4.20, too. There might be other constellations triggering it, but I haven't seen any yet. Juergen
