[Bug 2002429] Re: Patch needs to be removed ("debian/patches/gcc-ibmz-plt-revert.diff: Revert PLT changes from the gcc-11 branch")

[Chris Halse Rogers]

Hm, maybe I've been unclear.

What I mean is: while the launchpad buildds don't use gcc-11 for
anything in kinetic onwards, launchpad buildds aren't the only users of
a compiler and the Ubuntu kernel is not the only thing that users of a
compiler might compile.

gcc-11 is a package available in the archive, and users can use it¹,
regardless of the particular rationale that *we* have for including it
in the archive. If a jammy user is deliberately using gcc-11 I would
expect that upgrading to kinetic would not regress any fixes we've made
to the jammy package.

I don't *think* we have a policy that non-default compilers are not
subject to the usual SRU policy? If we do, maybe that should be better
communicated :)

Again, I'm OK with deciding to tell users of gcc-11 that to get this fix
they need to either stay on Jammy or upgrade all the way to Lunar. I
just want to make sure that we've actively decided to tell users this is
unsupported, rather than just verifying that it doesn't affect *us*.

¹: Maybe they're working around a gcc-12 miscompilation, or don't want
to revalidate gcc-12 just now, or just like the number 11.

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https://bugs.launchpad.net/bugs/2002429

Title:
  Patch needs to be removed ("debian/patches/gcc-ibmz-plt-revert.diff:
  Revert PLT changes from the gcc-11 branch")

Status in Ubuntu on IBM z Systems:
  New
Status in gcc-11 package in Ubuntu:
  Fix Released
Status in gcc-11-cross package in Ubuntu:
  New
Status in gcc-11-cross-ports package in Ubuntu:
  New
Status in gcc-11 source package in Jammy:
  Fix Committed
Status in gcc-11-cross source package in Jammy:
  Fix Committed
Status in gcc-11-cross-ports source package in Jammy:
  Fix Committed
Status in gcc-11 source package in Kinetic:
  New
Status in gcc-11 source package in Lunar:
  Fix Released
Status in gcc-11 package in Debian:
  Fix Released

Bug description:
  [ Impact ]

   * Bug #1954676 (https://bugs.launchpad.net/bugs/1954676) was opened
  because of kernel soft lockup issues with Jammy 5.15. A temporary
  patch to gcc-11 was identified and released under debian/patches/gcc-
  ibmz-plt-revert.diff, which reverted gcc commit 0990d93 ("IBM Z: Use
  @PLT symbols for local functions in 64-bit mode"). An eventual fix to
  the kernel lockup was released under the upstream Linux kernel, commit
  f3b7e73b2c66.

   * To this end, the debian/patches/gcc-ibmz-plt-revert.diff patch
  should be removed from gcc-11 as it presents issues with object file
  symbols. An attempt at this is in my PPA -
  https://launchpad.net/~john-cabaj/+archive/ubuntu/gcc-11). This
  allowed me to get past my issues with the object file symbols.

  [ Test Plan ]

   * use kpatch-build testcases to build and load a livepatch on s390x
  kernel built with gcc-11

   * it should be successful

  [ Where problems could occur ]

   * Whilst the kernel issues was worked-around by reverting compiler
  changes, the actual issue was in the kernel code; which has since been
  fixed. In ubuntu the kernel has been fixed since Ubuntu-5.15.0-20.20

  [ Other Info ]
   
   * A bit sad that this was not detected earlier, and compiler fixed up earlier.

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