The problem is that a SELECT on one thread is blocked by INSERT ... ON
DUPLICATE KEY UPDATE on another thread even when low_priority_updates is
activated.

The solution is to possibly downgrade the lock type to the setting of
low_priority_updates if the INSERT cannot be concurrent.


sql/sql_insert.cc:
  Possibly downgrade lock type to the the setting of low_priority_updates if
  if the INSERT cannot be concurrent.