Processus
Lock/Unlock

-module (lock).
-export ([create/0, lock/1, unlock/1, guard/2]).

guard (Owner, N) ->
    receive
        {lock, From} when N == 0 ->
            From ! ok,
            guard (From, 1);
        {lock, Owner} ->
            Owner ! ok,
            guard (Owner, N+1);
        {unlock, Owner} when N > 0 ->
            guard (Owner, N-1)
    end.

create () -> spawn (?MODULE, guard, [none, 0]).

lock (Lock) ->
    Lock ! {lock, self()},
    receive
        ok -> ok
    end.

unlock (Lock) -> Lock ! {unlock, self()}.