Model verifying that wakeup really

can be called after release without
causing deadlock.

sleep1.p

@@ -0,0 +1,134 @@
+/*
+This file defines a Promela model for xv6's
+acquire, release, sleep, and wakeup, along with
+a model of a simple producer/consumer queue.
+
+To run:
+	spinp sleep1.p
+
+(You may need to install Spin, available at http://spinroot.com/.)
+
+After a successful run spin prints something like:
+
+	unreached in proctype consumer
+		(0 of 37 states)
+	unreached in proctype producer
+		(0 of 23 states)
+
+After an unsuccessful run, the spinp script prints
+an execution trace that causes a deadlock.
+
+The safe body of producer reads:
+
+		acquire(lk);
+		x = value; value = x + 1; x = 0;
+		wakeup(0);
+		release(lk);
+		i = i + 1;
+
+If this is changed to:
+
+		x = value; value = x + 1; x = 0;
+		acquire(lk);
+		wakeup(0);
+		release(lk);
+		i = i + 1;
+
+then a deadlock can happen, because the non-atomic
+increment of value conflicts with the non-atomic 
+decrement in consumer, causing value to have a bad value.
+Try this.
+
+If it is changed to:
+
+		acquire(lk);
+		x = value; value = x + 1; x = 0;
+		release(lk);
+		wakeup(0);
+		i = i + 1;
+
+then nothing bad happens: it is okay to wakeup after release
+instead of before, although it seems morally wrong.
+*/
+
+#define ITER 4
+#define N 2
+
+bit lk;
+byte value;
+bit sleeping[N];
+
+inline acquire(x)
+{
+	atomic { x == 0; x = 1 }
+}
+
+inline release(x)
+{
+	assert x==1;
+	x = 0
+}
+
+inline sleep(cond, lk)
+{
+	assert !sleeping[_pid];
+	if
+	:: cond ->
+		skip
+	:: else ->
+		atomic { release(lk); sleeping[_pid] = 1 };
+		sleeping[_pid] == 0;
+		acquire(lk)
+	fi
+}
+
+inline wakeup()
+{
+	w = 0;
+	do
+	:: w < N ->
+		sleeping[w] = 0;
+		w = w + 1
+	:: else ->
+		break
+	od
+}
+
+active[N] proctype consumer()
+{
+	byte i, x;
+
+	i = 0;
+	do
+	:: i < ITER ->
+		acquire(lk);
+		sleep(value > 0, lk);
+		x = value; value = x - 1; x = 0;
+		release(lk);
+		i = i + 1;
+	:: else ->
+		break
+	od;
+	i = 0;
+	skip
+}
+
+active[N] proctype producer()
+{
+	byte i, x, w;
+
+	i = 0;
+	do
+	:: i < ITER ->
+		acquire(lk);
+		x = value; value = x + 1; x = 0;
+		release(lk);
+		wakeup();
+		i = i + 1;
+	:: else ->
+		break
+	od;
+	i = 0;
+	skip	
+}
+

spinp

@@ -0,0 +1,16 @@
+#!/bin/sh
+
+if [ $# != 1 ] || [ ! -f "$1" ]; then
+	echo 'usage: spinp file.p' 1>&2
+	exit 1
+fi
+
+rm -f $1.trail
+spin -a $1 || exit 1
+cc -DSAFETY -DREACH -DMEMLIM=500 -o pan pan.c
+pan -i
+rm pan.* pan
+if [ -f $1.trail ]; then
+	spin -t -p $1
+fi
+