It would seem to me that if you get an error from copyout in
sysctl_doeproc() you might return with the proclist_lock locked.

A simple diff is appended below.  Comments, anyone?

/assar

Index: kern_sysctl.c
===================================================================
RCS file: /cvsroot/syssrc/sys/kern/kern_sysctl.c,v
retrieving revision 1.55
diff -u -w -u -w -r1.55 kern_sysctl.c
--- kern_sysctl.c	1999/11/17 23:24:54	1.55
+++ kern_sysctl.c	2000/01/16 14:39:50
@@ -1013,11 +1013,11 @@
 			error = copyout((caddr_t)p, &dp->kp_proc,
 					sizeof(struct proc));
 			if (error)
-				return (error);
+				goto cleanup;
 			error = copyout((caddr_t)&eproc, &dp->kp_eproc,
 					sizeof(eproc));
 			if (error)
-				return (error);
+				goto cleanup;
 			dp++;
 			buflen -= sizeof(struct kinfo_proc);
 		}
@@ -1037,6 +1037,9 @@
 		*sizep = needed;
 	}
 	return (0);
+ cleanup:
+	proclist_unlock_read();
+	return (error);
 }
 
 /*