Task Creation and Semaphore Tutorial

[byteit101]

Unless you have fancy code, you should not encounter this (for the most part), but watch out for race conditions and locking.
if I have task A and it is running:

CRITICAL_REGION(Sem1)
Wait(5.0);//demo purposes
CRITICAL_REGION(Sem2)
/*Code here*/
END_REGION
END_REGION

and you have Task B running:

CRITICAL_REGION(Sem2)
Wait(5.0);//demo purposes
CRITICAL_REGION(Sem1)
/*Code here*/
END_REGION
END_REGION

Assuming they were started within 5 seconds of each other, they will both lock, and nothing will happen. Why?
Task A locks Sem1 and waits for 5 seconds, where Task B starts, and Locks Sem2
Task A has waited 5 seconds, and tries to lock Sem2, but Task B has it, so It waits for B to release it.
Meanwhile, B has waited its 5 seconds, and tries to Lock Sem1, but Task A has it so it waits for A to release it.
they are each waiting for the other to do something, so they do nothing

[taichichuan]

Be careful of using binary semaphores (via semBCreate) for mutual exclusion. There is a rather nasty problem known as priority inversion that can happen if you're not careful.

Binary semaphores are best used for synchronization like:

Forever() {
wait here until something happens;
do what must be done;
}

And in another piece of code:

signal that something must be done

That is, binary semaphores are usually found as a single instance (semTake in VxWorks or the synchronize method in the WPILib) and a semGive is located in another piece of code.

For mutual exclusion (i.e., to protect a piece of data from simultaneous access) the declaration would look more like:

m_mySemaphore = semMCreate(SEM_Q_PRIORITY | SEM_DELETE_SAFE | SEM_INVERSION_SAFE);

Then the use of the semaphore is more consistent with the WPILib

CRITICAL_REGION(m_mySemaphore);
...
END_REGION

construct.

Using a binary semaphore for mutual exclusion is what got NASA into trouble on Mars with the first rover (before Spirit, et al). They had to create a patch and use interpanetary FTP to upload it. Then they pressed the reset button from Earth and held their breath... Fortunately, it worked.

HTH,

Mike