VxWorks 学习笔记－Semaphores

§ Semaphores

1.Overview

Binary semaphores: Synchronization

• 申请、和释放由不同的任务执行。

• Caveat: if the event repeats too quickly, information may be lost.

Mutual exclusion semaphores: Mutual Exclusion

• 有owner的概念。

• 申请和释放由同一个任务(owner)执行。

• 一个任务可以申请多次。

• 只有owner才可以释放。

Counting semaphores

2.More about Mutual exclusion semaphores

Deletion Safety (owner exists)

• The deletion safety option prevents a task from being deleted while it owns the semaphore.

• SEM_DELETE_SAFE options

• STATUS taskDeleteForce (tid)

Unbounded Priority Inversion

• Problem exists

• Priority inheritance algorithm solves the unbounded priority inversion problem.

• Enabled on mutex semaphores by specifying the SEM_INVERSION_SAFE option during semMCreate( ).

编程建议

• 编写访问资源的库函数

• 在库函数中使用互斥机制

• 用户只通过库函数访问资源

Deadlock Problem

• Problem description
  

• Solutions: 1) 只使用一个信号量保护资源, 2) 所有任务按照相同的顺序访问资源

其他注意事项

• 不能在ISR 中使用；

• Critical region 尽量短；

Locking Out Preemption

• When doing something quick frequently, it may be preferable to disable preemption instead of taking a mutex.

• Does not disable interrupts.

• taskLock( )/taskUnlock( ) is faster than semGive( )/semTake( ).

ISR’s and Mutual Exclusion (ISR VS. Task)

• ISR’s can’t use mutex semaphores.

• Task sharing a resource with an ISR may need to disable interrupts.

• To disable/re-enable interrupts: int intLock( ), void intUnlock (lockKey)