Debug dump on large machine shows when many OpenMP threads (401 in total)
sleep on a barrier, one of the innermost nesting levels sleeps
on a child's b_arrived flag whose value is equal to 4 and is equal to
checker value. i.e., (1) sleep bit is 0, and (2) done_check() would
return true if called.
It is unclear how this might happen. It could be Windows Server 2016's
error of EnterCriticalSection / LeaveCriticalSection, or
error of WaitForSingleObject / SetEvent / ResetEvent, or
error in the library which is very difficult to find.
As a workaround, change INFINITE wait to timed wait, so that each
thread awakens each 5 seconds (the timeout was chosen arbitrary to not
disturb other threads much), check flag condition under the lock, and
either go to sleep again or stop sleeping as a result of the check.
Patch by Andrey Churbanov