• Anton Altaparmakov's avatar
    NTFS: Fix a nasty deadlock that appeared in recent kernels. · ba6d2377
    Anton Altaparmakov authored
    
          The situation: VFS inode X on a mounted ntfs volume is dirty.  For
          same inode X, the ntfs_inode is dirty and thus corresponding on-disk
          inode, i.e. mft record, which is in a dirty PAGE_CACHE_PAGE belonging
          to the table of inodes, i.e. $MFT, inode 0.
          What happens:
          Process 1: sys_sync()/umount()/whatever...  calls
          __sync_single_inode() for $MFT -> do_writepages() -> write_page for
          the dirty page containing the on-disk inode X, the page is now locked
          -> ntfs_write_mst_block() which clears PageUptodate() on the page to
          prevent anyone else getting hold of it whilst it does the write out.
          This is necessary as the on-disk inode needs "fixups" applied before
          the write to disk which are removed again after the write and
          PageUptodate is then set again.  It then analyses the page looking
          for dirty on-disk inodes and when it finds one it calls
          ntfs_may_write_mft_record() to see if it is safe to write this
          on-disk inode.  This then calls ilookup5() to check if the
          corresponding VFS inode is in icache().  This in turn calls ifind()
          which waits on the inode lock via wait_on_inode whilst holding the
          global inode_lock.
          Process 2: pdflush results in a call to __sync_single_inode for the
          same VFS inode X on the ntfs volume.  This locks the inode (I_LOCK)
          then calls write-inode -> ntfs_write_inode -> map_mft_record() ->
          read_cache_page() for the page (in page cache of table of inodes
          $MFT, inode 0) containing the on-disk inode.  This page has
          PageUptodate() clear because of Process 1 (see above) so
          read_cache_page() blocks when it tries to take the page lock for the
          page so it can call ntfs_read_page().
          Thus Process 1 is holding the page lock on the page containing the
          on-disk inode X and it is waiting on the inode X to be unlocked in
          ifind() so it can write the page out and then unlock the page.
          And Process 2 is holding the inode lock on inode X and is waiting for
          the page to be unlocked so it can call ntfs_readpage() or discover
          that Process 1 set PageUptodate() again and use the page.
          Thus we have a deadlock due to ifind() waiting on the inode lock.
          The solution: The fix is to use the newly introduced
          ilookup5_nowait() which does not wait on the inode's lock and hence
          avoids the deadlock.  This is safe as we do not care about the VFS
          inode and only use the fact that it is in the VFS inode cache and the
          fact that the vfs and ntfs inodes are one struct in memory to find
          the ntfs inode in memory if present.  Also, the ntfs inode has its
          own locking so it does not matter if the vfs inode is locked.
    Signed-off-by: default avatarAnton Altaparmakov <aia21@cantab.net>
    ba6d2377
mft.c 99.5 KB