1. Chapter 8: Mutual Exclusion Semaphores

2. Priority ceiling protocol
PCP is used in scheduling to avoid deadlock due to priority inversion.
In PCP, each resource is assigned a “priority ceiling”, which is a priority equal to the highest priority of any task which may lock the resource.
A process will not get scheduled if any resource it may lock actually has been locked by another process.

3. Priority ceiling protocol
When a task locks the resource, its priority is temporarily raised to the “priority ceiling”, thus no task that may lock the resource is able to get scheduled.
this allows a low priority task to defer execution of a higher-priority task
by doing this it prevents deadlock

4. Priority ceiling protocol
Task 1
Task 2
Task 3
Task 4

5. Mutual Exclusion Semaphores
A mutex is used by your application code to reduce the priority inversion problem.
The kernel can raise the priority of the lower priority task to shorten the blocking time suffered by the higher priority task.
In order to implement mutex, a kernel needs to provide the ability to support multiple tasks at the same priority. …

6. Example1 OSMutexCreate(VH, &err); void taskPrioM { while(1) {
void taskPrioH {
whle(1) {
/*…*/
OSMutexPend(Mutex, 0, &err);
OSMutexPost(Mutex); }
void taskPrioM {
while(1) {
/*…*/ }
void taskPrioL {
whle(1) {
/*…*/
OSMutexPend(Mutex, 0, &err);
OSMutexPost(Mutex); }

7. Example - without PIP H ready Lock(sem) continue M ready L ready
Unlock(sem)

8. Example - with PIP H ready Lock(sem) continue M ready L ready
Unlock(sem)

9. Example - μC/OS-II Sem = min(H, L) - 1 H ready Lock(sem) continue M
Unlock(sem)

10. Functions OSMutexCreate() OSMutexDel() OSMutexPend() OSMutexAccept()
OSMutexPost()
OSMutexQuery()

11. OSMutexCreate()
The major difference between OSMutexCreate() and OSSemCreate()
OSMutexCreate() would reserve a priority slot for priority inheritance protocol
OS_EVENT_TYPE_SEM
OS_EVENT_TYPE_SEM
0x00
???
PIP prio
0xFF
PIP prio
Task priority
(owner)
NULL
tcb_ptr
0x00 … 0x00
??? … ???
available
Owned by a task

12. OSEventWaitListInit;
OSMutexCreate()
input:
　prio
output: 　　
　OSEvent*
　Err
TskTbl[prio]
!= 1?
Reserve the priority;
get a ECB;
ECB.ower = AVALIABLE;
ECB.prio = prio;
OSEventWaitListInit;
retrun;

13. Reserve a priority by placing a non-null value
osmutexCreate()

14. OSMutexDel() opt =? DEL_ALWAYS DEL_NO_PEND EventGrp = 0 EventGrp = 0
EventTaskRdy
TskTbl[prio] =0
Free(Sem) N
EventGrp = 0
reterun err 14 Y

15. OSMutexDel()
Notice that to setup the field to UNUSED can prevent user from miss using the kernel object

16. OSMutexDel()

17. OSMutexPend() <<input>> OSEvent* timeout err
the resource is free Y
Acquire the resource
OSEvent->owner= cur
return; N
pri[cur] > pri[owner] N
Rdy Q
=> Waiting Q
Call scheduler Y …
return;
<<priority inheritance>>
//pri[owner] = pri[resource]
Update the TCB of the task “owner”
if (“owner” in the ready Q)
update the RdyTbl & RdyGrp

18. OSMutexPend()

20. OSMutexPost() <<input>> OSEvent* PIP == cur.Priority? N
Make the HPT in waiting Q ready to run
Call scheduler Y …
<<priority inheritance>>
Return to the original priority

21. OSMutexPost()

22. OSMutexPost()

23. OSMutexAccept()