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Concept
Critical section
Summary
In concurrent programming, concurrent accesses to shared resources can lead to unexpected or erroneous behavior, so 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, a peripheral device, or a network connection, that would not operate correctly in the context of multiple concurrent accesses.
Different codes or processes may consist of the same variable or other resources that need to be read or written but whose results depend on the order in which the actions occur. For example, if a variable is to be read by process A, and process B has to write to the same variable at the same time, process A might get either the old or new value of .
Process A:
// Process A
b = x + 5; // instruction executes at time = Tx
Process B:
// Process B
x = 3 + z; // instruction executes at time = Tx
In cases where a locking mechanism with finer granularity is not needed, a critical section is important. In the above case, if A needs to read the updated value of , executing process A, and process B at the same time may not give required results. To prevent this, variable is protected by a critical section. First, B gets the access to the section. Once B finishes writing the value, A gets the access to the critical section, and variable can be read.
By carefully controlling which variables are modified inside and outside the critical section, concurrent access to the shared variable are prevented. A critical section is typically used when a multi-threaded program must update multiple related variables without a separate thread making conflicting changes to that data. In a related situation, a critical section may be used to ensure that a shared resource, for example, a printer, can only be accessed by one process at a time.
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