I read on SO in an answer of a question: "A lock allows only one thread to enter the part that's locked" and many people have voted that up. The answer further scales up the definition to explain mutex and semaphore.
In another place (on SO as well as on many other sites) I saw piece of code that uses "critical section" to create read lock.
I believe they both are right. But I wonder if despite of being said in theory "read lock allows concurrent readers", in actual implementation it's still just single thread allowed at a time?