Comment 1 for bug 1191375

consider following scenario
* thread t1 is running
* t1 calls wait_begin -> wake_or_create_thread -> create_worker ->
                      -> mysql_thread_create -> spawn_thread_v1
  current thread's PFS_thread* remembered here as parent thread's
  PFS_thread* for newly created thread t2
* t1 finishes it's wait, sends result to client and finishes itself
* t2 is just about to start, it tries to attach instrumentation to
  itself. It wants to access PFS_thread* of t1 which is no longer
  available.
this scenario is impossible in Percona Server 5.5 as pointers
to parent thread's structures are not used
I'll try to reproduce it with DEBUG_SYNC or using other tricks