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  • Al Viro's avatar
    [PATCH] saner handling of auto_acct_off() and DQUOT_OFF() in umount · 7b7b1ace
    Al Viro authored
    The way we currently deal with quota and process accounting that might
    keep vfsmount busy at umount time is inherently broken; we try to turn
    them off just in case (not quite correctly, at that) and
      a) pray umount doesn't fail (otherwise they'll stay turned off)
      b) pray nobody doesn anything funny just as we turn quota off
    Moreover, LSM provides hooks for doing the same sort of broken logics.
    The proper way to deal with that is to introduce the second kind of
    reference to vfsmount.  Semantics:
     - when the last normal reference is dropped, all special ones are
       converted to normal ones and if there had been any, cleanup is done.
     - normal reference can be cloned into a special one
     - special reference can be converted to normal one; that's a no-op if
       we'd already passed the point of no return (i.e.  mntput() had
       converted special references to normal and started cleanup).
    The way it works: e.g. starting process accounting converts the vfsmount
    reference pinned by the opened file into special one and turns it back
    to normal when it gets shut down; acct_auto_close() is done when no
    normal references are left.  That way it does *not* obstruct umount(2)
    and it silently gets turned off when the last normal reference to
    vfsmount is gone.  Which is exactly what we want...
    The same should be done by LSM module that holds some internal
    references to vfsmount and wants to shut them down on umount - it should
    make them special and security_sb_umount_close() will be called exactly
    when the last normal reference to vfsmount is gone.
    quota handling is even simpler - we don't use normal file IO anymore, so
    there's no need to hold vfsmounts at all.  DQUOT_OFF() is done from
    deactivate_super(), where it really belongs.
    Signed-off-by: default avatarAl Viro <>
    Signed-off-by: default avatarLinus Torvalds <>