Взаимное исключение в многопоточном программировании

In computing, restricting data to be accessible by one thread at a time
the concept in logic and probability theory

In computer science, mutual exclusion is a property of concurrency control, which is instituted for the purpose of preventing race conditions. It is the requirement that one thread of execution never enters a critical section while a concurrent thread of execution is already accessing said critical section, which refers to an interval of time during which a thread of execution accesses a shared resource or shared memory. The shared resource is a data object, which two or more concurrent threads are trying to modify (where two concurrent read operations are permitted but, no two concurrent write operations or one read and one write are permitted, since it leads to data inconsistency). Mutual exclusion algorithms ensure that if a process is already performing write operation on a data object [critical section] no other process/thread is allowed to access/modify the same object until the first process has finished writing upon the data object [critical section] and released the object for other processes to read and write upon. The requirement of mutual exclusion was first identified and solved by Edsger W. Dijkstra in his seminal 1965 paper "Solution of a problem in concurrent programming control", which is credited as the first topic in the study of concurrent algorithms. A simple example of why mutual exclusion is important in practice can be visualized using a singly linked list of four items, where the second and third are to be removed. The removal of a node that sits between two other nodes is performed by changing the next pointer of the previous node to point to the next node (in other words, if node i is being removed, then the next pointer of node i – 1 is changed to point to node i + 1, thereby removing from the linked list any reference to node i). When such a linked list is being shared between multiple threads of execution, two threads of execution may attempt to remove two different nodes simultaneously, one thread of execution changing the next pointer of node i – 1 to point to node i + 1, while another thread of execution changes the next pointer of node i to point to node i + 2. Although both removal operations complete successfully, the desired state of the linked list is not achieved: node i + 1 remains in the list, because the next pointer of node i – 1 points to node i + 1. This problem (called a race condition) can be avoided by using the requirement of mutual exclusion to ensure that simultaneous updates to the same part of the list cannot occur. The term mutual exclusion is also used in reference to the simultaneous writing of a memory address by one thread while the aforementioned memory address is being manipulated or read by one or more other threads.

On uni processor systems, the simplest solution to achieve mutual exclusion is to disable interrupts during a process's critical section. This will prevent any interrupt service routines from running (effectively preventing a process from being preempted). Although this solution is effective, it leads to many problems. If a critical section is long, then the system clock will drift every time a critical section is executed because the timer interrupt is no longer serviced, so tracking time is impossible during the critical section. Also, if a process halts during its critical section, control will never be returned to another process, effectively halting the entire system. A more elegant method for achieving mutual exclusion is the busy wait. Busy waiting is effective for both uniprocessor and multiprocessor systems. The use of shared memory and an atomic test and set instruction provide the mutual exclusion. A process can test and set on a location in shared memory, and since the operation is atomic, only one process can set the flag at a time. Any process that is unsuccessful in setting the flag can either go on to do other tasks and try again later, release the processor to another process and try again later, or continue to loop while checking the flag until it is successful in acquiring it. Preemption is still possible, so this method allows the system to continue to function—even if a process halts while holding the lock. Several other atomic operations can be used to provide mutual exclusion of data structures; most notable of these is compare and swap (CAS). CAS can be used to achieve wait free mutual exclusion for any shared data structure by creating a linked list where each node represents the desired operation to be performed. CAS is then used to change the pointers in the linked list during the insertion of a new node. Only one process can be successful in its CAS; all other processes attempting to add a node at the same time will have to try again. Each process can then keep a local copy of the data structure, and upon traversing the linked list, can perform each operation from the list on its local copy.

One binary test&set register is sufficient to provide the deadlock free solution to the mutual exclusion problem. But a solution built with a test&set register can possibly lead to the starvation of some processes which become caught in the trying section.

Most algorithms for mutual exclusion are designed with the assumption that no failure occurs while a process is running inside the critical section. However, in reality such failures may be commonplace. For example, a sudden loss of power or faulty interconnect might cause a process in a critical section to experience an unrecoverable error or otherwise be unable to continue. If such a failure occurs, conventional, non failure tolerant mutual exclusion algorithms may deadlock or otherwise fail key liveness properties. To deal with this problem, several solutions using crash recovery mechanisms have been proposed.