RTEMS Ada User's Guide
Priority ceiling is an algorithm that calls for the lower priority task holding a resource to have its priority increased to that of the highest priority task which will EVER block waiting for that resource. This algorithm addresses the problem of priority inversion although it avoids the possibility of changing the priority of the task holding the resource multiple times. The priority ceiling algorithm will only change the priority of the task holding the resource a maximum of one time. The ceiling priority is set at creation time and must be the priority of the highest priority task which will ever attempt to acquire that semaphore.
RTEMS supports priority ceiling for local, binary semaphores that use the priority task wait queue blocking discipline. When a task of lower priority than the ceiling priority successfully obtains the semaphore, its priority is raised to the ceiling priority. When the task holding the task completely releases the binary semaphore (i.e. not for a nested release), the holder's priority is restored to the value it had before any higher priority was put into effect.
The need to identify the highest priority task which will attempt to obtain a particular semaphore can be a difficult task in a large, complicated system. Although the priority ceiling algorithm is more efficient than the priority inheritance algorithm with respect to the maximum number of task priority changes which may occur while a task holds a particular semaphore, the priority inheritance algorithm is more forgiving in that it does not require this apriori information.
The RTEMS implementation of the priority ceiling algorithm takes into account the scenario in which a task holds more than one binary semaphore. The holding task will execute at the priority of the higher of the highest ceiling priority or at the priority of the highest priority task blocked waiting for any of the semaphores the task holds. Only when the task releases ALL of the binary semaphores it holds will its priority be restored to the normal value.
RTEMS Ada User's Guide
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