Store in QLock the pc of the holder.
Useful when you mess things up and forget to release a lock.

Reference: /n/sources/patch/applied/qlock-pc
Date: Sat May 11 21:45:50 CES 2013
Signed-off-by: nemo@lsub.org

--- /sys/src/9/port/portdat.h	Sat May 11 21:44:51 2013
+++ /sys/src/9/port/portdat.h	Sat May 11 21:44:46 2013
@@ -108,6 +108,7 @@
 	Proc	*head;		/* next process waiting for object */
 	Proc	*tail;		/* last process waiting for object */
 	int	locked;		/* flag */
+	uintptr	qpc;		/* pc of the holder */
 };
 
 struct RWlock
--- /sys/src/9/port/qlock.c	Sat May 11 21:44:57 2013
+++ /sys/src/9/port/qlock.c	Sat May 11 21:44:54 2013
@@ -29,6 +29,7 @@
 	rwstats.qlock++;
 	if(!q->locked) {
 		q->locked = 1;
+		q->qpc = getcallerpc(&q);
 		unlock(&q->use);
 		return;
 	}
@@ -46,6 +47,7 @@
 	up->qpc = getcallerpc(&q);
 	unlock(&q->use);
 	sched();
+	q->qpc = getcallerpc(&q);
 }
 
 int
@@ -58,6 +60,7 @@
 		return 0;
 	}
 	q->locked = 1;
+	q->qpc = getcallerpc(&q);
 	unlock(&q->use);
 	return 1;
 }
@@ -81,6 +84,7 @@
 		return;
 	}
 	q->locked = 0;
+	q->qpc = 0;
 	unlock(&q->use);
 }