Peterson's 1981 two-process algorithm showed that mutual exclusion could be expressed with only ordinary shared variables—provided the memory model supplied the ordering guarantees the proof assumes.
Leslie Lamport's 1974 bakery algorithm showed that mutual exclusion could be achieved using ordinary shared reads and writes, even under surprisingly weak assumptions about overlapping memory operations.
Edsger Dijkstra's semaphore abstraction gave concurrent programs explicit P and V operations for mutual exclusion, resource counting and process coordination.