Why does this multi-threaded code print 6 some of the time?

I think I have figured out the sequence of events leading to this issue:

Thread 1 enters if (_state == 3)

Context switch

Thread 2 enters if (_state == 3)
Thread 2 increments state (state = 4)

Context switch

Thread 1 reads _state as 4

Context switch

Thread 2 sets _state = 3
Thread 2 enters if (_state == 3)

Context switch

Thread 1 executes _state = 4 + 1

Context switch

Thread 2 reads _state as 5
Thread 2 executes _state = 5 + 1;