Re: [gentoo-server] upgrading to gcc 3.4.4 on production server

[Kerin Millar]

On 14/02/06, Timo Veith <tv@×××××××××××.de> wrote:
> Hi list readers,
>
> I have a server on which I haven't done the upgrade yet because I am
> afraid of that potentially dangerous and long running procedure.
>
> What experiences do you have with upgrading gcc on production systems?

I have done it.

> Did everything went smoothly by following the upgrade howto [1]?

Yes, I've kept one system alive through well over 2 years worth of
upgrades (gcc and all). I actually upgraded to gcc-3.4 before a HOWTO
existed. A lot of fuss has been made about the topic in general but
really - once you cut through all the noise - it only boils down to
two points:

  1) Make sure packages which are written (or partially written) in
C++ are rebuilt (i.e. they should all link to libstdc++.so.6
afterwards, not libstdc++.so.5).

  2) It's also a good idea to rebuild your kernel/modules with the
newer compiler. The reason is that your loadable kernel modules and
the kernel image itself should be built with the same compiler (if you
forget to ensure this is the case, you'll notice it when trying to
build loadable kernel modules using ebuilds or if you try to augment
an existing kernel with additional in-tree modules).

There is no requirement to rebuild everything, just as there has never
been with previous gcc versions. The aspect that makes the migration
from gcc-3.3 -> gcc-3.4 so tricky is the libstdc++.so incompatibility.
If you remove libstdc++.so.5 before any C++ binaries have been rebuilt
then they'll break. This library is provided by *either* gcc-3.3 (and
lower) OR the sys-libs/libstdc++-v3 package. That's why the HOWTO
tells you to install libstdc++-v3 early. It makes the following kind
of operation safe for example:

  # emerge libstdc++-v3 && emerge -C =gcc-3.3*

Provided that the newer gcc version is selected and properly activated
(with gcc-config) any C++ stuff that's rebuilt will link to
libstdc++.so.6. It's important to attend to these packages if you have
any because, whilst you can continue to run them in their present
state so long as libstdc++.so.5 is provided, you can end up with
compiliation/linking issues further down the line. So, I recommend
actually taking a good look at listing 3.2 in the HOWTO and
determining what's effected and attending to that *first*, even if you
intend to rebuild everything in the long run.

Once the C++ stuff has been dealt with there's actually no need for
the older C++ runtime (on my system I actually never bothered
installing libstdc++-v3 and placed a reference in
/etc/portage/profiles/package.provided). I don't necessarily recommend
this for anyone else because some folks use binary-only packages which
implicitly link to libstdc++.so.5 and we have no option to recompile
them although they are few and far between.

With regards to my second point about the kernel, remember that the
kernel is self-hosted and it's not mandatory to rebuild it. But let's
say you have a kernel image currently executing which was built with
gcc-3.3 but you then try to make modifications your existing source
tree at /usr/src/linux and then (re)build it accordingly ... or that
you try to emerge something that builds a loadable module against that
tree. That's when it causes problems so what I would suggest is to
make a backup of the "/usr/src/linux/.config" file then run "make
mrproper" in the tree to clean it up. Then simply copy the config file
back in and build over, given that gcc-3.4 is the active compiler and
reboot with the rebuild kernel as and when it's convenient.

Finally, I'd like to say that I've personally seen a LOT of people
coming into #gentoo on IRC who have somehow managed to break python
(which links to libstdc++.so) and thus cannot emerge anything. In view
of how many people I have had to attend to in this regard, I would
very strongly recommend to anyone who is about to upgrade to do one of
the following two things prior:

  1) echo "dev-lang/python nocxx" >> /etc/portage/package.use &&
emerge --oneshot python (you can revert the flag and rebuild later if
you want)

  2) echo "=dev-lang/python-2.4.2-r1" >> /etc/portage/package.keywords
&& emerge --oneshot python (because this revision removes the
pointless linkage to libstdc++.so)

Either of those steps will render you immune to this particular
problem. If you wish to return python to its previous "state" after
the upgrade than that's obviously fine.

HTH,

--Kerin

-- 
gentoo-server@g.o mailing list