alt Hacker NewsNo, such algorithms exist and they use various mechanisms to achieve this. For larger data structures a common trick is to make a copy, update the copy, and then replace the original or parts of it with the copy. This provide readers with a stable view of the data structure that does not depend on small atomic reads. Another mechanism is that the different threads help each other to complete their interrupted work instead of making it invalid by modifying the data right away.
Not true.
If a writer writes continuously the shared data, it is impossible for the other thread to make the copy that must be edited.
If the copy succeeds, then you are right that an updated version could be substituted to the original using an atomic operation on pointers.
But there is no way to guarantee that the first copy succeeds.
Of course, in practice RCU is used very frequently, because all the other threads are well behaved and access the shared data for a minimum time, so the copy will succeed in most cases.
But absolute guarantees are impossible inside an algorithm expressible in an abstract programming language. Only using functions of the operating system to detect and stop a misbehaving thread can solve all cases.