user
47 lines
1.6 KiB
Markdown
47 lines
1.6 KiB
Markdown
# K11 — Mutex, LockGuard and priority inheritance
|
||
|
||
## Position
|
||
|
||
- Series: FreeRTOS C++
|
||
- Lesson: L10, card K11
|
||
- Duration: 30 minutes
|
||
- Actors: LOW=1, MEDIUM=2, HIGH=3, coordinator=4
|
||
- Storage: static mutex control and static task stacks
|
||
|
||
## Outcome
|
||
|
||
The student distinguishes mutex ownership from semaphore signaling, repairs
|
||
early returns with `LockGuard`, and explains every state/priority transition in
|
||
a measured priority-inheritance schedule.
|
||
|
||
## Lesson plan
|
||
|
||
| Time | Mode | Evidence |
|
||
| --- | --- | --- |
|
||
| 0–5 | ownership | owner-only give; ordinary mutex non-recursive |
|
||
| 5–10 | LockGuard | three returns each give exactly once |
|
||
| 10–15 | setup | LOW takes; HIGH attempts; MEDIUM ready |
|
||
| 15–20 | inheritance | HIGH blocked; LOW priority rises 1→3 |
|
||
| 20–25 | release | MEDIUM has not run; HIGH acquires next |
|
||
| 25–30 | restore | LOW returns to 1; protected counter is 11 |
|
||
|
||
## Acceptance
|
||
|
||
- mutex handle equals caller static control and heap delta is zero;
|
||
- LOW owns at first checkpoint and HIGH owns after handoff;
|
||
- HIGH is blocked while waiting;
|
||
- LOW priority is exactly 1, then 3, then 1;
|
||
- MEDIUM does not run before LOW release;
|
||
- event order proves inherited LOW precedes MEDIUM;
|
||
- guard restores on every host early-return path;
|
||
- ordinary recursive attempt fails;
|
||
- target exits with PASS and shared counter 11.
|
||
|
||
## Main traps
|
||
|
||
1. A binary semaphore is not an ownership mutex and has no inheritance.
|
||
2. Ordinary mutex recursion deadlocks/fails; use a recursive type deliberately.
|
||
3. Only the owner may unlock.
|
||
4. Mutex APIs are not ISR APIs.
|
||
5. Inheritance is temporary and bounded by the held mutex lifetime.
|