Messages in this thread | | | Date | Tue, 16 Oct 2018 10:01:50 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86: modify inline asm constraints in __cmpxchg_double() |
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* Juergen Gross <jgross@suse.com> wrote:
> On 16/10/2018 08:25, Ingo Molnar wrote: > > > > * Juergen Gross <jgross@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 <jgross@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?
Should we propagate the fix/enhancement to x86/urgent? It's currently in x86/asm for a v4.20 merge.
Thanks,
Ingo
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