[Bug 1899800] Re: Runtime deadlock: pthread_cond_signal failed to wake up pthread_cond_wait due to a bug in undoing stealing

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** Tags added: patch

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Title:
  Runtime deadlock: pthread_cond_signal failed to wake up
  pthread_cond_wait due to a bug in undoing stealing

Status in glibc package in Ubuntu:
  New

Bug description:
  This bug was submitted by Qin Li to glibc bugzilla earlier this year,
  with a one-line patch, though it hasn't been merged into glibc yet:

  https://sourceware.org/bugzilla/show_bug.cgi?id=25847

  This bug in pthread conditions will deadlock the OCaml runtime, as
  well as Python's runtime, and .NET.

  The bug was introduced in glibc 2.27, so affects Ubuntu 18.04 onwards.
  I confirm my OCaml app, as well as the repro from the bugzilla,
  deadlocks on Ubuntu 20.04 and Ubuntu 18.04.  To further strengthen the
  case that this is because of a bug in glibc, my app and the repro do
  not deadlock on Ubuntu 16.04.

  To rule out kernel issues, I further confirm that no deadlock happens
  when I copy Ubuntu 16.04's libc to 18.04 and redirect the dynamic
  linker so my app loads the earlier libc.

  I confirm that the one-line patch (available at the above bugzilla)
  applies cleanly on top of:

  * glibc-2.31-0ubuntu9.1 (Ubuntu 20.04 latest)
  * glibc-2.28-10 (Debian Buster/10 latest)
  * glibc-2.27-3ubuntu1.2 (Ubuntu 18.04 latest)

  I confirm that the one-line patch to glibc cures the deadlock issue in
  my OCaml apps.

  On Ubuntu 20.04, I have not been able to get the repro to deadlock in
  5 days.  My OCaml apps have not deadlocked in 5 days.

  On Debian Buster/10, the repro has not deadlocked in about 5 days.
  This is my desktop box, and I can otherwise use normal applications as
  usual like the GNOME environment, etc.

  On Ubuntu 18.04, the repro takes about 24-48 hours before it triggers
  a deadlock.  Prior to patching glibc, it would take only a few hours.
  I have not seen my OCaml apps deadlock since applying this patch,
  however.

  On Ubuntu 16.04 I have not been able to get the repro to deadlock
  ever.  My OCaml apps never deadlocked on this platform.  This is
  expected, since this platform runs glibc 2.23, where the bug has not
  been introduced yet (the bugzilla report claims introduced in 2.27).

  As for why 18.04 still deadlocks, I suspect another, unrelated pthread
  bug was introduced in glibc 2.27 and fixed by 2.28.  When applied to
  glibc 2.27, the one-line patch appears to significantly reduce the
  deadlocking by an order of magnitude.

  Please kindly consider merging the patch into Ubuntu glibc.

  More background about this bug, for the sake of future internet searchers:
  * https://discuss.ocaml.org/t/is-there-a-known-recent-linux-locking-bug-that-affects-the-ocaml-runtime

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