[Bug 1642390] Please test proposed package

[Steve Langasek]

Hello Adam, or anyone else affected,

Accepted glibc into yakkety-proposed. The package will build now and be
available at https://launchpad.net/ubuntu/+source/glibc/2.24-3ubuntu2 in
a few hours, and then in the -proposed repository.

Please help us by testing this new package.  See
https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how
to enable and use -proposed.Your feedback will aid us getting this
update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug,
mentioning the version of the package you tested, and change the tag
from verification-needed to verification-done. If it does not fix the
bug for you, please add a comment stating that, and change the tag to
verification-failed.  In either case, details of your testing will help
us make a better decision.

Further information regarding the verification process can be found at
https://wiki.ubuntu.com/QATeam/PerformingSRUVerification .  Thank you in
advance!

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https://bugs.launchpad.net/bugs/1642390

Title:
  Disable lock-elision in glibc pending upstream changes

Status in glibc package in Ubuntu:
  New
Status in glibc source package in Xenial:
  Fix Committed
Status in glibc source package in Yakkety:
  Fix Committed

Bug description:
  [ SRU Justification ]
  A long thread on debian-devel[1] has pointed out that hardware lock elision behaves differently (arguably, more correctly) in the face of incorrect pthread usage, especially where double-unlocks are concerned.  While this isn't itself a bug, it's quite true that we have no way of knowing right now just how many third party sources are buggy, and as people are upgrading to hardware that supports this feature, more software may crash in the future.

  The long-term goal should be to try to ferret out all the bad actors
  here, fix them, and re-enable hardware lock elision as the default,
  but the sane conservative choice seems to be to just disable the
  feature entirely for now, while we (the glibc upstream community) work
  on making lock elision a per-process opt-in.

  [1] https://lists.debian.org/debian-devel/2016/11/msg00210.html

  [ Proposed Fix ]
  This SRU will disable lock elision entirely, thus avoiding the problematic codepaths.  A future SRU will re-enable building those codepaths, but in such a way that they're disabled by default with a per-process environment twiddle to enable it on demand.

  [ Regression Potential ]
  There's a potential for performance regressions, however given that most x86 users are only just now upgrading to hardware that supports this feature, the status quo is that this codepath is dead to most users.  As for s390x and PPC, we've had requests to disable lock elision by default on both arches (entirely on s390x, conditionally on PPC), and those requests will be somewhat satisfied by this SRU, and cleaned up in the future SRU where we integrate the upstream-concensus patch to allow it to be conditionally re-enabled.

  [ Testing ]
  autopkgtest regressions will be our guide here on if this SRU is causing any issues.

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