One key point to notice in Figure 3-3 is that the control structure must be acquired. There is no such thing as a valiant try or everyone is a winner when asking for the control structure. You either successfully acquire the control structure or you do not. And you cannot execute the kernel code to access the control structure's associated memory structure until you have acquired the control structure.

Both latches and mutexes can be used to pin a memory structure. An object is pinned to ensure it is not deallocated or removed. In contrast, a lock is used to prevent an inappropriate change. For example, if a buffer in the block buffer cache is being changed, it is first pinned to ensure another process does not replace the buffer with another block from disk. Multiple processes can pin the same object, and the object is considered pinned if at least one process has it pinned. Another example is related to a cursor in the library cache. When a SQL statement is being executed, its cursor is pinned to ensure its memory is not deallocated and reallocated for some other shared pool purpose. Pinning must occur extremely quickly.

* Oracle continues to improve latches and mutexes. For example, in Oracle Database 10g Release 2, Oracle uses less memory per latch.2