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Mutual exclusion, in computer science, refers to the problem of ensuring that no two processes or threads (henceforth referred to only as processes) can be in their critical section at the same time. Here, a critical section refers to a period of time when the process accesses a shared resource, such as shared memory. The problem of mutual exclusion was first identified and solved by Edsger W. Dijkstra in his seminal 1965 paper titled: Solution of a problem in concurrent programming control.
A simple example of why mutual exclusion is important in practice can be visualized using a singly linked list. (See Figure 1.) In such a linked list the removal of a node is done by changing the “next” pointer of the preceding node to point to the subsequent node (e.g., if node i is being removed then the “next” pointer of node i-1 will be changed to point to node i+1). In an execution where such a linked list is being shared between multiple processes, two processes may attempt to remove two different nodes simultaneously resulting in the following problem: let nodes i and i+1 be the nodes to be removed; furthermore, let neither of them be the head nor the tail; the next pointer of node i-1 will be changed to point to node i+1 and the next pointer of node i will be changed to point to node i+2. Although both removal operations complete successfully, node i+1 remains in the list since i-1 was made to point to i+1 skipping node i (which was made to point to i+2). This can be seen in the Figure 1. This problem can be avoided using mutual exclusion to ensure that simultaneous updates to the same part of the list cannot occur.
There are both software and hardware solutions for enforcing mutual exclusion. Some different solutions are discussed below.
On uniprocessor systems, the simplest solution to achieve mutual exclusion is to disable interrupts during a process' 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 since 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-wait is effective for both uniprocessor and multiprocessor systems. The use of shared memory and an atomic test-and-set instruction provides 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. This method allows the system to continue to function even if a process halts while holding the lock since preemption is still possible.
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. This can be achieved 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.
Beside the hardware supported solution, some software solutions exist that use "busy-wait" to achieve the goal. Examples of these include the following:
Spin locks and busy waiting take up excessive processor time and power and are considered anti-patterns in almost every case. In addition, these algorithms do not work if out-of-order execution is utilized on the platform that executes them. Programmers have to specify strict ordering on the memory operations within a thread.
The solution to these problems is to use synchronization facilities provided by an operating system's multithreading library, which will take advantage of hardware solutions if possible but will use software solutions if no hardware solutions exist. For example, when the operating system's lock library is used and a thread tries to acquire an already acquired lock, the operating system will suspend the thread using a context switch and swaps it out with another thread that is ready to be run, or could put that processor into a low power state if there is no other thread that can be run. Therefore, most modern mutual exclusion methods attempt to reduce latency and busy-waits by using queuing and context switches. However, if the time that is spent suspending a thread and then restoring it can be proven to be always more than the time that must be waited for a thread to become ready to run after being blocked in a particular situation, then spinlocks are a fine solution for that situation only.
Synchronization primitives can be built like the examples below by using the solutions explained above:
Many forms of mutual exclusion have side-effects. For example, classic semaphores permit deadlocks, in which one process gets a semaphore, another process gets a second semaphore, and then both wait forever for the other semaphore to be released. Other common side-effects include starvation, in which a process never gets sufficient resources to run to completion, priority inversion in which a higher priority thread waits for a lower-priority thread, and "high latency" in which response to interrupts is not prompt.
Much research is aimed at eliminating the above effects, such as by guaranteeing non-blocking progress. No perfect scheme is known.
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