Minor kernel changes.

kern/vfs/vfslist.c

@@ -129,6 +129,23 @@ vfs_biglock_acquire(void)
 	if (!lock_do_i_hold(vfs_biglock)) {
 		lock_acquire(vfs_biglock);
 	}
+	else if (vfs_biglock_depth == 0) {
+		/*
+		 * Supposedly we hold it, but the depth is 0. This may
+		 * mean: (1) the count is messed up, or (2)
+		 * lock_do_i_hold is lying. Since OS/161 ships out of
+		 * the box with unimplemented locks (students
+		 * implement them) that always return true, assume
+		 * situation (2). In this case acquire the lock
+		 * anyway.
+		 *
+		 * Once you have working locks, this won't be the
+		 * case, and if you get here it should be situation
+		 * (1), in which case the count is messed up and one
+		 * can panic.
+		 */
+		lock_acquire(vfs_biglock);
+	}
 	vfs_biglock_depth++;
 }
 
@@ -381,6 +398,9 @@ vfs_doadd(const char *dname, int mountable, struct device *dev, struct fs *fs)
 	unsigned index;
 	int result;
 
+	/* Silence warning with gcc 4.8 -Og (but not -O2) */
+	index = 0;
+
 	vfs_biglock_acquire();
 
 	name = kstrdup(dname);

kern/vfs/vfslookup.c

@@ -135,6 +135,13 @@ getdevice(char *path, char **subpath, struct vnode **startvn)
 
 	KASSERT(vfs_biglock_do_i_hold());
 
+	/*
+	 * Entirely empty filenames aren't legal.
+	 */
+	if (path[0] == 0) {
+		return EINVAL;
+	}
+
 	/*
 	 * Locate the first colon or slash.
 	 */

kern/vm/kmalloc.c

@@ -1065,6 +1065,10 @@ subpage_kfree(void *ptr)
 
 	checksubpages();
 
+	/* Silence warnings with gcc 4.8 -Og (but not -O2) */
+	prpage = 0;
+	blktype = 0;
+
 	for (pr = allbase; pr; pr = pr->next_all) {
 		prpage = PR_PAGEADDR(pr);
 		blktype = PR_BLOCKTYPE(pr);