Sorts this using func.
The sort function func is passed two elements of the this. It should return 0 if they are
equal, a negative value if the first element should be higher in the this or a positive value if the
first element should be lower in the this than the second element.
This function must be called while holding the this's lock.
| this | |
| func |
the CompareDataFunc is used to sort this |
| user_data |
user data passed to |