[Bug 1642390] Update Released

[Adam Conrad]

The verification of the Stable Release Update for glibc has completed
successfully and the package has now been released to -updates.
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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:
  Confirmed
Status in glibc source package in Xenial:
  Fix Released
Status in glibc source package in Yakkety:
  Fix Released

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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