Книга: Real-Time Concepts for Embedded Systems

15.2.1 Resource Synchronization

15.2.1 Resource Synchronization

Access by multiple tasks must be synchronized to maintain the integrity of a shared resource. This process is called resource synchronization, a term closely associated with critical sections and mutual exclusions.

Mutual exclusion is a provision by which only one task at a time can access a shared resource. A critical section is the section of code from which the shared resource is accessed.

As an example, consider two tasks trying to access shared memory. One task (the sensor task) periodically receives data from a sensor and writes the data to shared memory. Meanwhile, a second task (the display task) periodically reads from shared memory and sends the data to a display. The common design pattern of using shared memory is illustrated in Figure 15.1.


Figure 15.1: Multiple tasks accessing shared memory.

Problems arise if access to the shared memory is not exclusive, and multiple tasks can simultaneously access it. For example, if the sensor task has not completed writing data to the shared memory area before the display task tries to display the data, the display would contain a mixture of data extracted at different times, leading to erroneous data interpretation.

The section of code in the sensor task that writes input data to the shared memory is a critical section of the sensor task. The section of code in the display task that reads data from the shared memory is a critical section of the display task. These two critical sections are called competing critical sections because they access the same shared resource.

A mutual exclusion algorithm ensures that one task's execution of a critical section is not interrupted by the competing critical sections of other concurrently executing tasks.

One way to synchronize access to shared resources is to use a client-server model, in which a central entity called a resource server is responsible for synchronization. Access requests are made to the resource server, which must grant permission to the requestor before the requestor can access the shared resource. The resource server determines the eligibility of the requestor based on pre-assigned rules or run-time heuristics.

While this model simplifies resource synchronization, the resource server is a bottleneck. Synchronization primitives, such as semaphores and mutexes, and other methods introduced in a later section of this chapter, allow developers to implement complex mutual exclusion algorithms. These algorithms in turn allow dynamic coordination among competing tasks without intervention from a third party.

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