[all-commits] [llvm/llvm-project] c2c431: [OpenMP] Fix bug 50022

[Shilei Tian via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: c2c43132f69c6f7e89d97c0c2ce32b0cf2f30e90
      https://github.com/llvm/llvm-project/commit/c2c43132f69c6f7e89d97c0c2ce32b0cf2f30e90
  Author: Shilei Tian <tianshilei1992 at gmail.com>
  Date:   2021-07-23 (Fri, 23 Jul 2021)

  Changed paths:
    A openmp/libomptarget/test/offloading/bug50022.cpp
    M openmp/runtime/src/kmp_tasking.cpp

  Log Message:
  -----------
  [OpenMP] Fix bug 50022

Bug 50022 [0] reports target nowait fails in certain case, which is added in this
patch. The root cause of the failure is, when the second task is created, its
parent's `td_incomplete_child_tasks` will not be incremented because there is no
parallel region here thus its team is serialized. Therefore, when the initial
thread is waiting for its unfinished children tasks, it thought there is only
one, the first task, because it is hidden helper task, so it is tracked. The
second task will only be pushed to the queue when the first task is finished.
However, when the first task finishes, it first decrements the counter of its
parent, and then release dependences. Once the counter is decremented, the thread
will move on because its counter is reset, but actually, the second task has not
been executed at all. As a result, since in this case, the main function finishes,
then `libomp` starts to destroy. When the second task is pushed somewhere, all
some of the structures might already have already been destroyed, then anything
could happen.

This patch simply moves `__kmp_release_deps` ahead of decrement of the counter.
In this way, we can make sure that the initial thread is aware of the existence
of another task(s) so it will not move on. In addition, in order to tackle
dependence chain starting with hidden helper thread, when hidden helper task is
encountered, we force the task to release dependences.

Reference:
[0] https://bugs.llvm.org/show_bug.cgi?id=50022

Reviewed By: AndreyChurbanov

Differential Revision: https://reviews.llvm.org/D106519