Linux kernel locking mechanisms and shared resources

A resource is said to be shared when it is accessible by several contenders, whether exclusively or not. When it is 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, and the contenders might be processors, processes, or threads. The operating system performs mutual exclusion by atomically 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. Atomicity guarantees the modification to be entirely successful, or not successful at all. Modern operating systems nowadays rely on hardware (which should allow atomic operations) to implement synchronization, though a simple system may ensure atomicity by disabling interrupts (and avoiding scheduling) around the critical code section.

We can enumerate two synchronization mechanisms...