1. Resource Access Protocols
張軒彬助理教授
中興大學資訊科學系

2. Outline
Priority Inversion
Priority Inheritance Protocol

3. Resource A resource is any software structure
A variable, a memory area, a file, etc.
A resource may be shared or private.
Exclusive resource
A shared resource that is protected against concurrent access
A piece of code executed under mutual exclusion to guarantee exclusive resource accesses is called critical section
A resource is any software structure, for example, a data structure, a variable, man memory area or a file.
A resource may be shared or private. If shared, it may also be protected against concurrent accesses, which is called exclusive resource.

4. Priority Inversion
A high-priority job can experience two kinds of blocking
Bounded
For example, a high-priority job waits for the executions of a low-priority job to acquire a resource
The blocking time is no longer than that of the critical section where the lower priority job owns the resources
Necessary to ensure resource consistence
Unbounded
When a third job preempts the low-priority job during the inversion, thus delaying the high-priority job even more
Would lead to uncontrolled blocking and cause high-priory job miss its deadline
Have to be solved

5. An Example of Priority Inversion: Bounded
normal execution
critical section
J1 blocked
J 1
J 2 t1 t2
Example of blocking on an exclusive resource.
Priority of J1 > Priority of J2
J1 has a higher priority than J2.
J2 start its execution first and then enter a critical section. During the execution of its critical section, J1 is activated and preempts J2 since it has a higher priority than J2. However, when J1 want to enter its critical section to access the exclusive shared resource, it is blocked since the source is currently hold by J2.
As a result, J2 resume again. After it finish its execution in the critical section, the exclusive shared resource is unlocked and J1 change from block state to the ready time. Thus, J1 preempt J2 and enter its critical section.

6. An Example of Priority Inversion: Unbounded
normal execution
Priority: J1 > J2 > J3
critical section
J1 and J3 share an exclusive resource
J 1 blocked
J 1
Three tasks J1, J2 and J3 have decreasing priority. Besides, J1 and J3 share an exclusive resource.
At first, J3 start execution and enter its critical section. During the execution of its critical section, J1 is activated and preempts J3. However, when J1 want to enter its critical section to access the exclusive shared resource, it is blocked since the resource is currently hold by J3. Thus, J1 is blocked and J3 resumes execution.
At time t4, J2 becomes ready and preempt J2. After it finish execution, J3 resume again. At time t6, J3 finish execution and finally J1 is ready and enter its critical section.
Thus, in the interval [t3, t6], a priority inversion is occurred. Note that, the maximum blocking time that J1 may experience is undetermined, which would lead to uncontrolled blocking time and miss its deadline.
J 2
J 3 t0 t1 t2 t3 t4 t5 t6 t7
An example of priority inversion.

7. Priority Inversion (Cont.)
Solution
Disallow preemption during the execution of all critical sections
Only appropriate for very short critical sections because it may create unnecessary blocking
Priority Inheritance Protocol
Priority Ceiling Protocol
There are some approaches to solve the priority inversion problem.
First, we may disallow preemption during the execution of all critical section. However, this would result in unnecessary blocking since a higher-priority task that does not want to enter its critical section will also be blocked.
The other approaches are priority inheritance protocol and priority ceiling protocol.

8. Priority Inheritance Protocol
When a job Ti is blocked by a lower priority job Tk, Ti transmits its priority to the Tk.
In general, a job inherits the highest priority of the jobs blocked by it.
Priority inheritance is transitive;
If J3 blocks J2, and J2 blocks J1, then J3 inherits the priority of J1 via J2.
Priority inheritance protocol: modify the priority of host task that cause blocking.

9. Example Example of Priority Inheritance Protocol. normal execution
Priority: J1 > J2 > J3
critical section
J1 and J3 share an exclusive resource
Direct blocking
J 1
Push-through blocking
J 2
At time t3, which is called direct blocking, J1 want to enter its critical section but is blocked. J3 inheritance J1’s prioirty.
At time t4, which is called push-through blocking, J2 becomes ready, but however, is blocked.
J 3 t0 t1 t2 t3 t4 t5 t6 t7 P1 P3 P3
Example of Priority Inheritance Protocol.

10. Example Direct blocking Push-through blocking
Occurs when a higher-priority job tries to acquire a resource already held by a lower-priority job
Necessary to ensure the consistency of the shared resource
Push-through blocking
Occurs when a medium-priority job is blocked by a lower-priority job that has inherited a higher priority from a job it directly blocks
Necessary to avoid unbounded priority inversion

11. Priority Inheritance Protocol (Cont.)
Unsolved problems
Chained blocking
The blocking duration for a job can still be substantial because a chain of blocking can be formed
Deadlock
The protocol does not prevent deadlocks
However, the priority inheritance protocol may suffer from chained blocking and deadlock.

12. Example of chained blocking.
normal execution
Priority: J1 > J2 > J3
J1 and J3 share an exclusive resource a
critical section
J1 and J2 share an exclusive resource b
J 1 a b
J 2 b b
J1 have to wait for both J2 and J3 to release the shared resources a and b.
J 3 a a
Example of chained blocking.

13. Chained Blocking (Cont.)
From previous slide
J1 is blocked for the duration of two critical sections
Once to wait J3 to release resource a
Then to wait J2 to release resource b
This is called chain blocking
In the worst case, if J1 accesses n distinct resource that have been locked by n lower-priority jobs
J1 will be blocked for the duration of n critical sections

14. Deadlock normal execution critical section J 1 a J 2 b b
Blocked on Sb
wait(S a)
wait(S b)
J 1 a
wait(S b)
wait(S a)
Blocked on Sa
J 2
Two jobs that use two semaphores in a nested fashion but in reverse order.
As shown in the figure, J1 is blocked at t4. Then, J2 is blocked in t5. As a result, a deadlock is occurred. b b
signal(S b)
signal(S b) t1 t2 t3 t4 t5
signal(S a)
signal(S b)
Example of deadlock.

15. Deadlock (Cont.)
Two jobs that use two semaphores in a nested fashion but in reverse order
J1 is blocked on Sb at time t4. Then, J2 is blocked on Sa at time t5
Deadlock

16. Deadlock (Cont.)
Notably, the deadlock does not depend on the Priority Inheritance Protocol
But is caused by an erroneous use of semaphores
The deadlock problem can be solved by the mechanisms introduced in the Operating System
E.g., impose an total ordering on the resource access
Both the chained blocking and deadlock are be solved by the Protocol Ceiling Protocol. However, we skip it in our course.

17. Reference
Giorgio C. Buttazzo, “Hard Real-Time Computing Systems: Predictable Scheduling Algorithms and Applications,” Kluwer Academic Publishers, 1997
Jane W. S. Liu, “Real-Time Systems,” Prentice Hall, 2002