Re: clang fails on linux-next since commit 8bf705d13039

[Nick Desaulniers <ndesaulniers@google.com>]

 From the trace, looking at the source of __raw_cmpxchg(), it immediately
reminds me of this clang bug [0]

where Clang validates inline asm constraints before running dead code
elimination.

[0] https://bugs.llvm.org/show_bug.cgi?id=33587

On Sun, Mar 18, 2018 at 11:43 PM Dmitry Vyukov <dvyukov@google.com> wrote:

> On Sat, Mar 17, 2018 at 2:13 PM, Lukas Bulwahn <lukas.bulwahn@gmail.com>
wrote:
> > Hi Dmitry, hi Ingo,
> >
> > since commit 8bf705d13039 ("locking/atomic/x86: Switch atomic.h to use
atomic-instrumented.h")
> > on linux-next (tested and bisected from tag next-20180316), compiling
the
> > kernel with clang fails with:
> >
> > In file included from arch/x86/entry/vdso/vdso32/vclock_gettime.c:33:
> > In file included from arch/x86/entry/vdso/vdso32/../vclock_gettime.c:15:
> > In file included from ./arch/x86/include/asm/vgtod.h:6:
> > In file included from ./include/linux/clocksource.h:13:
> > In file included from ./include/linux/timex.h:56:
> > In file included from ./include/uapi/linux/timex.h:56:
> > In file included from ./include/linux/time.h:6:
> > In file included from ./include/linux/seqlock.h:36:
> > In file included from ./include/linux/spinlock.h:51:
> > In file included from ./include/linux/preempt.h:81:
> > In file included from ./arch/x86/include/asm/preempt.h:7:
> > In file included from ./include/linux/thread_info.h:38:
> > In file included from ./arch/x86/include/asm/thread_info.h:53:
> > In file included from ./arch/x86/include/asm/cpufeature.h:5:
> > In file included from ./arch/x86/include/asm/processor.h:21:
> > In file included from ./arch/x86/include/asm/msr.h:67:
> > In file included from ./arch/x86/include/asm/atomic.h:279:
> > ./include/asm-generic/atomic-instrumented.h:295:10: error: invalid
output size for constraint '=a'
> >                 return arch_cmpxchg((u64 *)ptr, (u64)old, (u64)new);
> >                        ^
> > ./arch/x86/include/asm/cmpxchg.h:149:2: note: expanded from macro
'arch_cmpxchg'
> >         __cmpxchg(ptr, old, new, sizeof(*(ptr)))
> >         ^
> > ./arch/x86/include/asm/cmpxchg.h:134:2: note: expanded from macro
'__cmpxchg'
> >         __raw_cmpxchg((ptr), (old), (new), (size), LOCK_PREFIX)
> >         ^
> > ./arch/x86/include/asm/cmpxchg.h:95:17: note: expanded from macro
'__raw_cmpxchg'
> >                              : "=a" (__ret), "+m" (*__ptr)
  \
> >                                      ^
> >
> > (... and some more similar and closely related errors)


> Thanks for reporting, Lukas.

> +more people who are more aware of the current state of clang for kernel.

> Are there are known issues in '=a' constraint handling between gcc and
> clang? Is there a recommended way to resolve them?

> Also, Lukas what's your version of clang? Potentially there are some
> fixes for kernel in the very latest versions of clang.



--
Thanks,
~Nick Desaulniers