Teach me about critical section in operating system, kernal
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A critical section is a segment of code that accesses shared resources in an operating system. When multiple processes need to access the same shared resource, such as a file or memory location, coordination becomes essential. Without proper synchronization, processes can interfere with each other, leading to incorrect results.
A race condition is a critical problem that occurs when multiple processes access shared data concurrently without proper synchronization. The final result depends on the unpredictable timing of process execution. For example, if two processes read the same variable simultaneously and then both write back modified values, one update may be lost.
Any solution to the critical section problem must satisfy three fundamental requirements. First is mutual exclusion - only one process can execute in the critical section at any given time. Second is progress - if no process is currently in the critical section, the decision of which process enters next cannot be postponed indefinitely. Third is bounded waiting - there must be a limit on how many times other processes can enter the critical section before a waiting process gets its turn.
Peterson's Algorithm is a elegant software solution for the critical section problem between two processes. It uses two shared variables: a flag array to indicate each process's intention to enter the critical section, and a turn variable that gives priority to the other process. When a process wants to enter its critical section, it first sets its flag to true, then sets turn to favor the other process, and finally waits until either the other process is not interested or it's not the other process's turn.
Modern processors provide hardware support for implementing critical sections efficiently. The Test-and-Set instruction atomically reads the current value of a memory location and sets it to true, returning the previous value. This atomic operation cannot be interrupted, making it ideal for implementing locks. Compare-and-Swap is another atomic operation that compares a memory location with an expected value and updates it only if they match. These hardware primitives provide the foundation for efficient synchronization mechanisms in modern operating systems.