Semaphore Issue

[Kirspel, Kevin]

I have a situation with semaphores that I thought someone may have a quick explanation for.  The scenario consists of 2 semaphore/mutexes and priority inheritance.  The first semaphore is created by the application (S1) and is of type: RTEMS_BINARY_SEMAPHORE | RTEMS_PRIORITY | RTEMS_INHERIT_PRIORITY.  The second (S2) is the allocator lock mutex that is used by malloc().  My scenario is as follows.

Task 1 has priority 36.  Task 2 has priority 34.

1.       Task 1 obtains S1.

2.       Task 1 obtains S2

3.       Task 2 preempts Task 1.

4.       Task 2 tries to obtain S2 but can't get it.  Task 1 priority is changed to 34.

5.       Task 1 begins running again.  Task 1 releases S2.  Task 1 priority is not changed back to 36 and continues to run.

6.       Task 1 continues operating for 3 more seconds.

7.       Task 1 releases S1.  Task 1 priority is now changed back to 36.

8.       Task 2 now preempts Task 1 and begins operating again.

Task 1 is the main user of Semaphore S1.  During normal operation no other task will try to obtain it.  It is there to protect against some custom shell commands that may access the same data.  Semaphore S2 is widely used since it protects memory allocation.  If obtaining/releasing S1 is removed from the scenario above then everything works OK.


1.       Task 1 obtains S2

2.       Task 2 preempts Task 1.

3.       Task 2 tries to obtain S2 but can't get it.  Task 1 priority is changed to 34.

4.       Task 1 begins running again.  Task 1 releases S2.  Task 1 priority is now changed back to 36.

5.       Task 2 now preempts Task 1 and begins operating again.

6.       Task 2 enters wait condition.

7.       Task 1 continues operating for 3 more seconds and then enters wait condition.

I am going to try and duplicate this scenario using a small test program but I wanted to see if anyone had any ideas of what may be causing the issue.  I have a mixed RTEMS source base between versions 4.10 and 4.11.   I started with 4.10 and upgraded just enough to get USB support.

Kevin Kirspel
Senior Research Engineer
Opti Medical
235 Hembree Park Drive
Roswell GA 30076
Tel: (770)-510-4444 ext. 81642
Direct: (770)-688-1642
Fax: (770)-510-4445

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