Updating Scala collections thread-safely

i was wondering if there is any 'easy' way to update immutable scala collections safely. Consider following code:

class a {
   private var x = Map[Int,Int]()

   def update(p:(Int,Int)) { x = x + (p) }
}

This code is not thread safe, correct? By that i mean that if we have two threads invoking update method and lets say that x is map containing { 1=>2 } and thread A invokes update((3,4)) and only manages to execute the x + (p) part of the code. Then rescheduling occurs and thread B invokes update((13,37)) and successfully updates the variable x. The thread A continues and finishes.

After all this finishes, value x would equal map containing { 1=>2, 3=>4 }, correct? Instead of desired { 1=>2, 3=>4, 13=>37 }. Is there a simple way to fix that? I hope it's undestandable what I'm asking :)

Btw, i know there are solutions like Akka STM but i would prefer not to use those, unless necessary.

Thanks a lot for any answer!

edit: Also, i would prefer solution without locking. Eeeew :)