INtime® 3.1 Software - tenAsys

Chapter 3: About INtime software’s RT kernel
Regions
A region is a single-unit semaphore with special suspension, deletion, and priorityadjustment
features. Regions provide mutual exclusion for resources or data; only one
thread may control a region at a time; only the thread in control of the region can
access the resource or data protected by a region. Once a thread gains control of a
region, the thread cannot be suspended or deleted until it gives up control of the
region. When the running thread no longer needs access, it exits the region, which
enables a waiting thread to obtain control of the region and thus access the resource or
data protected by that region.
Regions have a thread queue where threads wait for access to the region. The queue
may be FIFO- or priority-based.
Priority inversions
Regions also have a priority-inversion avoidance mechanism when the region’s thread
queue is priority based.
Then, if a higher-priority thread tries to enter a busy region, the priority of the thread in
the region is raised temporarily so that it equals the waiting thread's priority. This
helps prevent priority-inversion, as shown in this example:
1. Thread A is the running thread. It is a low-priority thread with control of a region,
accessing some data. The region has a priority queue. The only other thread that
uses the data is thread C, a high-priority thread that is not ready to run.
2. Thread B, a medium-priority thread, becomes ready to run and preempts A.
3. Thread C becomes ready to run and preempts B. It runs until it tries to gain control
of the region. Thread A's priority is raised to equal thread C's priority until thread
A releases the region; then its priority returns to its initial level.
4. When thread A releases the region, thread C receives control of the region and uses
the data. When thread C completes, thread B runs.
Without the priority inversion avoidance mechanism, thread B would have preempted
A while A had control of the region; C would have preempted B, but would have been
unable to use the data because A had control of the region.
Deadlocks
Regions require careful programming to avoid deadlock, where threads need
simultaneous access to resources protected by nested regions, and one thread has
control of one region while the other thread has control of another. To avoid deadlock,
all threads must access nested regions in the same, arbitrary order, and release them in
the same reverse order.
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