[BUG] ntfs: possible data races in ntfs_clear_extent_inode()

[Tuo Li <islituo@gmail.com>]

Hello,

Our static analysis tool finds some possible data races in the NTFS file
system in Linux 6.4.0-rc6.

In most calling contexts, the variable ni->ext.base_ntfs_ino is accessed
with holding the lock ni->extent_lock. Here is an example:

   ntfs_extent_mft_record_free() --> Line 2773 in fs/ntfs/mtf.c
     mutex_lock(&ni->extent_lock); --> Line 2786 in fs/ntfs/mtf.c (Lock 
ni->extent_lock)
     base_ni = ni->ext.base_ntfs_ino; --> Line 2787 in fs/ntfs/mft.c 
(Access ni->ext.base_ntfs_ino)

However, in the following calling contexts:

   ntfs_evict_big_inode() --> Line 2247 in fs/ntfs/inode.c
      ntfs_clear_extent_inode() --> Line 2274 in fs/ntfs/inode.c
         if (!is_bad_inode(VFS_I(ni->ext.base_ntfs_ino))) --> Line 2224 
in fs/ntfs/inode.c (Access ni->ext.base_ntfs_ino)

   ntfs_evict_big_inode() --> Line 2247 in fs/ntfs/inode.c
     ni->ext.base_ntfs_ino = NULL; --> Line 2285 in fs/ntfs/inode.c 
(Access ni->ext.base_ntfs_ino)

the variable ni->ext.base_ntfs_ino is accessed without holding the lock
ni->extent_lock, and thus data races can occur.

I am not quite sure whether these possible data races are real and how 
to fix them if they are real.
Any feedback would be appreciated, thanks!

Reported-by: BassCheck <bass@buaa.edu.cn>

Best wishes,
Tuo Li