On Thu, Jun 11, 2009 at 04:16:06PM -0400, Jon Hylands wrote:
> 
> Hi everyone,
> 
> I've got a couple USB-based serial ports on my robot BrainBot, and I've got
> four different processes - two for each port. One process reads from the
> serial port, and one writes to it.
> 
> For each serial port, I have a mutex semaphore to protect access to the
> port, like this:
> 
> readData
> 	^portAccess critical: [serialPort readByteArray]
> 
> writeData: aByteArray
> 	portAccess critical: [serialPort nextPutAll: aByteArray]
> 
> Those two methods are implemented in the same class, and thus the
> 'portAccess' instance variable (which is a Semaphore) is shared between
> read and write, but not between different serial ports..
> 
> I have two instances of this class, one for each USB device plugged in (the
> USB devices are both FT232-based interfaces, one at port 4, the other at
> port 5).
> 
> The question is, do I need to protect the two instances of SerialPort with
> a single Semaphore? Looking at the serial port code for read/writing, it
> looks like the code just jumps directly into the primitives. I was under
> the assumption that primitive execution can't be pre-empted, and therefore
> the code shouldn't need mutex protection.
> 
> Am I missing something?

It does not sound like you are missing anything. Are you encountering
problems with this, or are you just asking to be sure?

Dave