ext4: protect group inode free counting with group lock

commit 6f2e9f0 upstream.

Now when we set the group inode free count, we don't have a proper
group lock so that multiple threads may decrease the inode free
count at the same time. And e2fsck will complain something like:

Free inodes count wrong for group MiCode#1 (1, counted=0).
Fix? no

Free inodes count wrong for group MiCode#2 (3, counted=0).
Fix? no

Directories count wrong for group MiCode#2 (780, counted=779).
Fix? no

Free inodes count wrong for group MiCode#3 (2272, counted=2273).
Fix? no

So this patch try to protect it with the ext4_lock_group.

btw, it is found by xfstests test case 269 and the volume is
mkfsed with the parameter
"-O ^resize_inode,^uninit_bg,extent,meta_bg,flex_bg,ext_attr"
and I have run it 100 times and the error in e2fsck doesn't
show up again.

Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/ext4/ialloc.c

@@ -772,6 +772,8 @@ struct inode *ext4_new_inode(handle_t *handle, struct inode *dir, umode_t mode,
  ext4_itable_unused_set(sb, gdp,
  (EXT4_INODES_PER_GROUP(sb) - ino));
  up_read(&grp->alloc_sem);
+ } else {
+ ext4_lock_group(sb, group);
  }
  ext4_free_inodes_set(sb, gdp, ext4_free_inodes_count(sb, gdp) - 1);
  if (S_ISDIR(mode)) {
@@ -784,8 +786,8 @@ struct inode *ext4_new_inode(handle_t *handle, struct inode *dir, umode_t mode,
  }
  if (EXT4_HAS_RO_COMPAT_FEATURE(sb, EXT4_FEATURE_RO_COMPAT_GDT_CSUM)) {
  gdp->bg_checksum = ext4_group_desc_csum(sbi, group, gdp);
- ext4_unlock_group(sb, group);
  }
+ ext4_unlock_group(sb, group);
 
  BUFFER_TRACE(inode_bitmap_bh, "call ext4_handle_dirty_metadata");
  err = ext4_handle_dirty_metadata(handle, NULL, inode_bitmap_bh);