Re: Linux 2.6.27.27

[Krzysztof Oledzki]

On Tue, 21 Jul 2009, Krzysztof Oledzki wrote:

> On Mon, 20 Jul 2009, Linus Torvalds wrote:
>
> > On Mon, 20 Jul 2009, Krzysztof Oledzki wrote:
> > > No problem. Please let me know what should I do to help tracking this 
> > > issue.
> >
> > Can you build two kernels: one with -fwrapv, and one with
> > -fno-strict-overflow, and then verify that
> >
> > - they are otherwise identical (ie exact same source code, same compiler
> >   etc)
> >
> > - verify that yes, the -fwrapv kernel works, the other does not. Just to
> >   avoid the confusion that obviously exists with Debian/sid binutils
> >   upgrades that _also_ happens result in nonbootable kernels.
> >
> > - upload the 'vmlinux' images somewhere (I'm not sure what the limits for
> >   binary attachments are at the kernel bugzilla, but that would be the
> >   logical place)
> >
> > In fact, it would be nice to have a third "identical" kernel build, except
> > with neither -fwrapv/-fno-strict-overflow.
>
> OK. Right now I'm building the three kernels you asked for 
> (fwrapv/fno-strict-overflow/none). I'll test them and upload vmlinux images.

OK, there are three kernels, exactly as you requested:

http://noc.axelspringer.pl/no-strict-overflow-vs-wrapv/vmlinux-fno-strict-overflow.bz2 (Hangs)
http://noc.axelspringer.pl/no-strict-overflow-vs-wrapv/vmlinux-fwrapv.bz2 (OK)
http://noc.axelspringer.pl/no-strict-overflow-vs-wrapv/vmlinux-fnone.bz2 (OK)

# grep -A1 "can't wrap" linux-2.6.27.27-*/Makefile
linux-2.6.27.27-fno-strict-overflow/Makefile:# disable invalid "can't wrap" optimzations for signed / pointers
linux-2.6.27.27-fno-strict-overflow/Makefile-KBUILD_CFLAGS      += $(call cc-option,-fno-strict-overflow)
--
linux-2.6.27.27-fwrapv/Makefile:# disable invalid "can't wrap" optimzations for signed / pointers
linux-2.6.27.27-fwrapv/Makefile-KBUILD_CFLAGS   += $(call cc-option,-fwrapv)
--
linux-2.6.27.27-fnone/Makefile:# disable invalid "can't wrap" optimzations for signed / pointers
linux-2.6.27.27-fnone/Makefile-#KBUILD_CFLAGS   += $(call cc-option,-fno-strict-overflow)

Plwase note the third one has the KBUILD_CFLAGS commented.

Kernels are identical and are compiled from the same config, on the same 
server with gcc-4.2.4, binutils-2.19. There is no ccache installed and the 
kernels are not patched with any additonal patches - just vanilla 
linux-2.6.27.27.

Screenshot from the hanging kernel (-fno-strict-overflow):
 http://noc.axelspringer.pl/no-strict-overflow-vs-wrapv/2.6.27.27-hang.png

Dmesg from a bootable kernel:
 http://noc.axelspringer.pl/no-strict-overflow-vs-wrapv/dmesg

Best regards,

				Krzysztof Olędzki