switch fat to ->s_d_op, close exportfs races there

don't bother with lock_super() in fat_fill_super() callers, while
we are at it - there won't be any concurrency anyway.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/fat/fat.h

@@ -319,7 +319,8 @@ extern struct inode *fat_build_inode(struct super_block *sb,
 			struct msdos_dir_entry *de, loff_t i_pos);
 extern int fat_sync_inode(struct inode *inode);
 extern int fat_fill_super(struct super_block *sb, void *data, int silent,
-			const struct inode_operations *fs_dir_inode_ops, int isvfat);
+			const struct inode_operations *fs_dir_inode_ops,
+			int isvfat, void (*setup)(struct super_block *));
 
 extern int fat_flush_inodes(struct super_block *sb, struct inode *i1,
 		            struct inode *i2);

fs/fat/inode.c

@@ -703,7 +703,6 @@ static struct dentry *fat_fh_to_dentry(struct super_block *sb,
 		struct fid *fid, int fh_len, int fh_type)
 {
 	struct inode *inode = NULL;
-	struct dentry *result;
 	u32 *fh = fid->raw;
 
 	if (fh_len < 5 || fh_type != 3)
@@ -748,10 +747,7 @@ static struct dentry *fat_fh_to_dentry(struct super_block *sb,
 	 * the fat_iget lookup again.  If that fails, then we are totally out
 	 * of luck.  But all that is for another day
 	 */
-	result = d_obtain_alias(inode);
-	if (!IS_ERR(result))
-		d_set_d_op(result, sb->s_root->d_op);
-	return result;
+	return d_obtain_alias(inode);
 }
 
 static int
@@ -799,8 +795,6 @@ static struct dentry *fat_get_parent(struct dentry *child)
 	brelse(bh);
 
 	parent = d_obtain_alias(inode);
-	if (!IS_ERR(parent))
-		d_set_d_op(parent, sb->s_root->d_op);
 out:
 	unlock_super(sb);
 
@@ -1244,7 +1238,8 @@ static int fat_read_root(struct inode *inode)
  * Read the super block of an MS-DOS FS.
  */
 int fat_fill_super(struct super_block *sb, void *data, int silent,
-		   const struct inode_operations *fs_dir_inode_ops, int isvfat)
+		   const struct inode_operations *fs_dir_inode_ops, int isvfat,
+		   void (*setup)(struct super_block *))
 {
 	struct inode *root_inode = NULL, *fat_inode = NULL;
 	struct buffer_head *bh;
@@ -1280,6 +1275,8 @@ int fat_fill_super(struct super_block *sb, void *data, int silent,
 	if (error)
 		goto out_fail;
 
+	setup(sb); /* flavour-specific stuff that needs options */
+
 	error = -EIO;
 	sb_min_blocksize(sb, 512);
 	bh = sb_bread(sb, 0);

fs/fat/namei_msdos.c

@@ -227,11 +227,7 @@ static struct dentry *msdos_lookup(struct inode *dir, struct dentry *dentry,
 	}
 out:
 	unlock_super(sb);
-	d_set_d_op(dentry, &msdos_dentry_operations);
-	dentry = d_splice_alias(inode, dentry);
-	if (dentry)
-		d_set_d_op(dentry, &msdos_dentry_operations);
-	return dentry;
+	return d_splice_alias(inode, dentry);
 
 error:
 	unlock_super(sb);
@@ -661,21 +657,16 @@ static const struct inode_operations msdos_dir_inode_operations = {
 	.getattr	= fat_getattr,
 };
 
-static int msdos_fill_super(struct super_block *sb, void *data, int silent)
+static void setup(struct super_block *sb)
 {
-	int res;
-
-	lock_super(sb);
-	res = fat_fill_super(sb, data, silent, &msdos_dir_inode_operations, 0);
-	if (res) {
-		unlock_super(sb);
-		return res;
-	}
-
+	sb->s_d_op = &msdos_dentry_operations;
 	sb->s_flags |= MS_NOATIME;
-	d_set_d_op(sb->s_root, &msdos_dentry_operations);
-	unlock_super(sb);
-	return 0;
+}
+
+static int msdos_fill_super(struct super_block *sb, void *data, int silent)
+{
+	return fat_fill_super(sb, data, silent, &msdos_dir_inode_operations,
+			     0, setup);
 }
 
 static struct dentry *msdos_mount(struct file_system_type *fs_type,

fs/fat/namei_vfat.c

@@ -772,13 +772,10 @@ static struct dentry *vfat_lookup(struct inode *dir, struct dentry *dentry,
 
 out:
 	unlock_super(sb);
-	d_set_d_op(dentry, sb->s_root->d_op);
 	dentry->d_time = dentry->d_parent->d_inode->i_version;
 	dentry = d_splice_alias(inode, dentry);
-	if (dentry) {
-		d_set_d_op(dentry, sb->s_root->d_op);
+	if (dentry)
 		dentry->d_time = dentry->d_parent->d_inode->i_version;
-	}
 	return dentry;
 
 error:
@@ -1066,24 +1063,18 @@ static const struct inode_operations vfat_dir_inode_operations = {
 	.getattr	= fat_getattr,
 };
 
-static int vfat_fill_super(struct super_block *sb, void *data, int silent)
+static void setup(struct super_block *sb)
 {
-	int res;
-
-	lock_super(sb);
-	res = fat_fill_super(sb, data, silent, &vfat_dir_inode_operations, 1);
-	if (res) {
-		unlock_super(sb);
-		return res;
-	}
-
 	if (MSDOS_SB(sb)->options.name_check != 's')
-		d_set_d_op(sb->s_root, &vfat_ci_dentry_ops);
+		sb->s_d_op = &vfat_ci_dentry_ops;
 	else
-		d_set_d_op(sb->s_root, &vfat_dentry_ops);
+		sb->s_d_op = &vfat_dentry_ops;
+}
 
-	unlock_super(sb);
-	return 0;
+static int vfat_fill_super(struct super_block *sb, void *data, int silent)
+{
+	return fat_fill_super(sb, data, silent, &vfat_dir_inode_operations,
+			     1, setup);
 }
 
 static struct dentry *vfat_mount(struct file_system_type *fs_type,