ext4: fix lockdep warning about recursive inode locking

Setting a large xattr value may require writing the attribute contents
to an external inode. In this case we may need to lock the xattr inode
along with the parent inode. This doesn't pose a deadlock risk because
xattr inodes are not directly visible to the user and their access is
restricted.

Assign a lockdep subclass to xattr inode's lock.

 ============================================
 WARNING: possible recursive locking detected
 4.12.0-rc1+ #740 Not tainted
 --------------------------------------------
 python/1822 is trying to acquire lock:
  (&sb->s_type->i_mutex_key#15){+.+...}, at: [<ffffffff804912ca>] ext4_xattr_set_entry+0x65a/0x7b0

 but task is already holding lock:
  (&sb->s_type->i_mutex_key#15){+.+...}, at: [<ffffffff803d6687>] vfs_setxattr+0x57/0xb0

 other info that might help us debug this:
  Possible unsafe locking scenario:

        CPU0
        ----
   lock(&sb->s_type->i_mutex_key#15);
   lock(&sb->s_type->i_mutex_key#15);

  *** DEADLOCK ***

  May be due to missing lock nesting notation

 4 locks held by python/1822:
  #0:  (sb_writers#10){.+.+.+}, at: [<ffffffff803d0eef>] mnt_want_write+0x1f/0x50
  #1:  (&sb->s_type->i_mutex_key#15){+.+...}, at: [<ffffffff803d6687>] vfs_setxattr+0x57/0xb0
  #2:  (jbd2_handle){.+.+..}, at: [<ffffffff80493f40>] start_this_handle+0xf0/0x420
  #3:  (&ei->xattr_sem){++++..}, at: [<ffffffff804920ba>] ext4_xattr_set_handle+0x9a/0x4f0

 stack backtrace:
 CPU: 0 PID: 1822 Comm: python Not tainted 4.12.0-rc1+ #740
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
 Call Trace:
  dump_stack+0x67/0x9e
  __lock_acquire+0x5f3/0x1750
  lock_acquire+0xb5/0x1d0
  down_write+0x2c/0x60
  ext4_xattr_set_entry+0x65a/0x7b0
  ext4_xattr_block_set+0x1b2/0x9b0
  ext4_xattr_set_handle+0x322/0x4f0
  ext4_xattr_set+0x144/0x1a0
  ext4_xattr_user_set+0x34/0x40
  __vfs_setxattr+0x66/0x80
  __vfs_setxattr_noperm+0x69/0x1c0
  vfs_setxattr+0xa2/0xb0
  setxattr+0x12e/0x150
  path_setxattr+0x87/0xb0
  SyS_setxattr+0xf/0x20
  entry_SYSCALL_64_fastpath+0x18/0xad
Signed-off-by: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/inode.c

@@ -4877,6 +4877,8 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
 	}
 	brelse(iloc.bh);
 	ext4_set_inode_flags(inode);
+	if (ei->i_flags & EXT4_EA_INODE_FL)
+		ext4_xattr_inode_set_class(inode);
 	unlock_new_inode(inode);
 	return inode;
 

fs/ext4/xattr.c

@@ -107,6 +107,13 @@ const struct xattr_handler *ext4_xattr_handlers[] = {
 #define EXT4_GET_MB_CACHE(inode)	(((struct ext4_sb_info *) \
 				inode->i_sb->s_fs_info)->s_mb_cache)
 
+#ifdef CONFIG_LOCKDEP
+void ext4_xattr_inode_set_class(struct inode *ea_inode)
+{
+	lockdep_set_subclass(&ea_inode->i_rwsem, 1);
+}
+#endif
+
 static __le32 ext4_xattr_block_csum(struct inode *inode,
 				    sector_t block_nr,
 				    struct ext4_xattr_header *hdr)
@@ -828,6 +835,7 @@ static struct inode *ext4_xattr_inode_create(handle_t *handle,
 		ea_inode->i_op = &ext4_file_inode_operations;
 		ea_inode->i_fop = &ext4_file_operations;
 		ext4_set_aops(ea_inode);
+		ext4_xattr_inode_set_class(ea_inode);
 		ea_inode->i_generation = inode->i_generation;
 		EXT4_I(ea_inode)->i_flags |= EXT4_EA_INODE_FL;
 

fs/ext4/xattr.h

@@ -196,3 +196,9 @@ static inline int ext4_init_security(handle_t *handle, struct inode *inode,
 	return 0;
 }
 #endif
+
+#ifdef CONFIG_LOCKDEP
+extern void ext4_xattr_inode_set_class(struct inode *ea_inode);
+#else
+static inline void ext4_xattr_inode_set_class(struct inode *ea_inode) { }
+#endif