IPC Timeouts

[Jonathan S. Shapiro]

On Thu, 2005-02-24 at 12:58 +0100, Espen Skoglund wrote:
> A reason why timeouts are currently specified within the IPC is that
> doing so enables a program to invoke an IPC and set a timeout in one
> atomic operation.

Just to expand on what Espen is saying, this issue is very real. In
EROS, the issue is implemented differently. An application that needs
such a timeout has a "watchdog thread" (I am translating approximately
into L4 terminology).

The timed IPC executes

	watchMe := true   /* shared memory */
	doIPC()
	watchMe := false
        cancelTimer(wait-object)

The watchdog thread executes

	call(wait-object, timeout)
	if (watchMe)
	   "stun" the main thread
           if the main thread is in the receiving state,
             set an error code and advance the PC
           else
             do nothing

Yes, this is complex, but remember that this case is really very very
rare.

I'm not sure if this is useful to the L4 discussion, but perhaps it will
suggest other solutions that may have appeal.