The kernel locking API and shared objects

A resource is said to be shared when it can be accessed by several contenders, regardless of their exclusively. When they are exclusive, access must be synchronized so that only the allowed contender(s) may own the resource. Such resources might be memory locations or peripheral devices, while the contenders might be processors, processes, or threads. Operating systems perform mutual exclusion by atomically (that is, by means of an operation that can be interrupted) modifying a variable that holds the current state of the resource, making this visible to all contenders that might access the variable at the same time. This atomicity guarantees that the modification will either be successful, or not successful at all. Nowadays, modern operating systems rely on the hardware (which should allow atomic operations) used for implementing synchronization, though a simple system may ensure atomicity by disabling interrupts (and avoiding scheduling)...