commit 088136aa02de6fa13061ef6f754071a5652fdabd
Author: DJ Delorie <dj@redhat.com>
Date:   Mon May 15 22:50:35 2023 -0400

    i386: Use pthread_barrier for synchronization on tst-bz21269
    
    So I was able to reproduce the hangs in the original source, and debug
    it, and fix it.  In doing so, I realized that we can't use anything
    complex to trigger the thread because that "anything" might also cause
    the expected segfault and force everything out of sync again.
    
    Here's what I ended up with, and it doesn't seem to hang where the
    original one hung quite often (in a tight while..end loop).  The key
    changes are:
    
    1. Calls to futex are error checked, with retries, to ensure that the
       futexes are actually doing what they're supposed to be doing.  In the
       original code, nearly every futex call returned an error.
    
    2. The main loop has checks for whether the thread ran or not, and
       "unlocks" the thread if it didn't (this is how the original source
       hangs).
    
    Note: the usleep() is not for timing purposes, but just to give the
    kernel an excuse to run the other thread at that time.  The test will
    not hang without it, but is more likely to test the right bugfix
    if the usleep() is present.