Real-Time Operating Systems

Priority inversion (2)
¿ Basic priority inheritance protocol:
1. A job J uses its assigned priority, unless it is in its CS
and blocks higher priority jobs
In which case, J inherits P H
, the highest priority of the
jobs blocked by J
When J exits the CS, it resumes the priority it had at
the point of entry into the CS
2. Priority inheritance is transitive
¿ Advantage:
¿ Transparent to scheduler
¿ Disadvantage:
¿ Deadlock possible
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Priority inversion (3)
¿ Priority ceiling protocol
1. A priority ceiling is assigned to each resource, which is
equal to the highest priority task that may use the
resource
2. The scheduler transfers that priority to any task that
accesses the resource
3. Job J is allowed to start a new CS only if its priority is
higher than all priority ceilings of all the semaphores
locked by jobs other than J
4. On completion, J’s priority switches to its normal value
¿ Advantage:
¿ tasks can share resources simply by changing their
priorities, thus eliminating the need for semaphores
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