Критическая секция в многопоточном программировании

Protected section of code that cannot be executed by more than one process at a time
In concurrent programming, concurrent accesses to shared resources can lead to unexpected or erroneous behavior. Thus, the parts of the program where the shared resource is accessed need to be protected in ways that avoid the concurrent access. One way to do so is known as a critical section or critical region. This protected section cannot be entered by more than one process or thread at a time; others are suspended until the first leaves the critical section. Typically, the critical section accesses a shared resource, such as a data structure, peripheral device, or network connection, that would not operate correctly in the context of multiple concurrent accesses.

The implementation of critical sections vary among different operating systems. A critical section will usually terminate in finite time, and a thread, task, or process must wait for a fixed time to enter it (bounded waiting). To ensure exclusive use of critical sections, some synchronization mechanism is required at the entry and exit of the program. A critical section is a piece of a program that requires mutual exclusion of access. As shown in the figure, in the case of mutual exclusion (mutex), one thread blocks a critical section by using locking techniques when it needs to access the shared resource, and other threads must wait their turn to enter the section. This prevents conflicts when two or more threads share the same memory space and want to access a common resource. Once the critical section is exited, and in some cases the scheduled quantum completed, the pending interrupt will be executed. The concept of scheduling quantum applies to "round robin" and similar scheduling policies. Since critical sections may execute only on the processor on which they are entered, synchronization is only required within the executing processor. This allows critical sections to be entered and exited at almost no cost. No inter processor synchronization is required. Only instruction stream synchronization is needed. Most processors provide the required amount of synchronization by interrupting the current execution state. This allows critical sections in most cases to be nothing more than a per processor count of critical sections entered. Performance enhancements include executing pending interrupts at the exit of all critical sections and allowing the scheduler to run at the exit of all critical sections. Furthermore, pending interrupts may be transferred to other processors for execution. Critical sections should not be used as a long lasting locking primitive. Critical sections should be kept short enough so they can be entered, executed, and exited without any interrupts occurring from the hardware and the scheduler. Kernel level critical sections are the base of the software lockout issue.

In parallel programming, the code is divided into threads. The read write conflicting variables are split between threads and each thread has a copy of them. Data structures such as linked lists, trees, and hash tables have data variables that are linked and cannot be split between threads; hence, implementing parallelism is very difficult. To improve the efficiency of implementing data structures, multiple operations such as insertion, deletion, and search can be executed in parallel. While performing these operations, there may be scenarios where the same element is being searched by one thread and is being deleted by another. In such cases, the output may be erroneous. The thread searching the element may have a hit, whereas the other thread may subsequently delete it. These scenarios will cause issues in the program running by providing false data. To prevent this, one method is to keep the entire data structure under critical section so that only one operation is handled at a time. Another method is locking the node in use under critical section, so that other operations do not use the same node. Using critical section, thus, ensures that the code provides expected outputs.

Critical sections also occur in code which manipulates external peripherals, such as I/O devices. The registers of a peripheral must be programmed with certain values in a certain sequence. If two or more processes control a device simultaneously, neither process will have the device in the state it requires and incorrect behavior will ensue. When a complex unit of information must be produced on an output device by issuing multiple output operations, exclusive access is required so that another process does not corrupt the datum by interleaving its own bits of output. In the input direction, exclusive access is required when reading a complex datum via multiple separate input operations. This prevents another process from consuming some of the pieces, causing corruption. Storage devices provide a form of memory. The concept of critical sections is equally relevant to storage devices as to shared data structures in main memory. A process which performs multiple access or update operations on a file is executing a critical section that must be guarded with an appropriate file locking mechanism.