bug in jthread scheduling mechanism?

[Mihai Surdeanu]

Godmar (or anybody familiar with the jthread internals),
I think I found a bug in the jthread scheduling mechanism which causes
kaffe to block. Below I attach an execution trace that blocks. Note: I
modified kaffe to work in a slave configuration: instead of the main
thread I have 3 threads in the trace below: a dispatcher thread
waiting for job requests on a socket and 2 workers that actually do the
jobs (D - dispatcher or main, W1, W2 - workers).
Kaffe blocks because "handleIO" tries to read from sigPipe twice even
though only one byte is written. Please go through the trace to understand
why it happens. I ignored everything not relevant to this problem in the
trace.

*************************************************************************

W2: 		send something on socket
W2:		wait on condvar for dispatcher signal
		reschedule other thread
...
W1: 		send something on socket 
W1: 		wait on condvar for dispatcher signal
SIGIO: 		IO reply for W2 arrives
		ints are blocked so write to sigPipe[1]
W1: 		enter reschedule, handleIO(true) (no runnable thread)
		add sigPipe[0] to pending read fds
		enter select with infinite wait time
		select returns 2 (sigPipe[0] and W2's reply)
		interrupted by SIGVTALRM
SIGVTALRM: 	reschedule the dispatcher D
D:		read from socket
		signal worker W2
W2:		do Java stuff
		send something on socket
		wait on condvar for IO reply
		enter reschedule, handleIO(true) 
			(no runnable thread, W1 still waiting in condvar)
                add sigPipe[0] to pending read fds
                enter select with infinite wait time
		select returns 1 (sigPipe still has a byte in it)
		EMPTY SIGPIPE!
		reenter handleIO(true)
		reenter select with infinite wait time
		select returns 1 (W1's reply)
SIGIO: 		reschedule the dispatcher D
D:		read from socket
                signal worker W1
W1:		(W1 is right after select in handleIO(true))
		select returned with FD_ISSET(sigPipe[0])
		try to read from sigPipe[0]
		BLOCK IN READ, sigPipe previously emptied by W2!	

*************************************************************************

To solve the problem I just made sigPipe[0] non-blocking using
"jthreadedFileDescriptor". 
Sorry I can't provide a real test program. My code is not stable enough to
be released. Nevertheless, one could write a test program just for this
problem, I just don't have the time for this.

Regards,
Mihai Surdeanu		Southern Methodist University, CSE 
			(214) 768 - 3054