[Bug 1560634] Re: [Ubuntu 16.04.1] Provide a way to dynamically enable lock elision on glibc

[Chris Feyerchak]

There's documentation at [1] of crashes in at least three different applications being avoided by disabling lock elision.  Rebuilding glibc is restrictive for a lot of the impacted users.  I'd be in favor of any of the following solutions:
a) disabling lock elision by default
b) providing a means of disabling lock elision at runtime
c) adding an alternative glibc *.deb to the repositories that can easily be installed with an "apt-get install" or similar.

[1] https://github.com/MyrtleSoftware/glibc-no-lock-elision

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Title:
  [Ubuntu 16.04.1] Provide a way to dynamically enable lock elision on
  glibc

Status in glibc package in Ubuntu:
  Triaged

Bug description:
  We identified that, in some scenarios, lock elision can cause performance regressions.
  In order to minimize this, we prepared a patch for POWER that disables lock elision by default even when glibc is built with --enable-lock-elision.
  It's enabled only when GLIBC_ELISION_ENABLE=yes is available in the environment and when the hardware supports it.

  This patch has already been sent upstream [1], but the community asked us to re-implement it based on the tunables framework.  It's now blocked waiting for the acceptance of the tunables framework [2] [3] [4].  The discussion around the tunables framework is already old and the community cannot agree yet on a way to pass the tuning parameters.
  We had an agreement it would be integrated into glibc 2.23, which didn't happen and due to other issues, we don't believe this will happen until glibc 2.24.

  Meanwhile, in order to solve the regression issues reported by current customers, would you consider to accept the following patch?
  It's very close to patch [1], which has already been refused upstream.

  [1] http://patchwork.sourceware.org/patch/8529/
  [2] http://patchwork.sourceware.org/patch/10405/
  [3] http://patchwork.sourceware.org/patch/10405/
  [4] http://patchwork.sourceware.org/patch/10342/

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