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  • Al Viro's avatar
    rcuwalk: recheck mount_lock after mountpoint crossing attempts · b37199e6
    Al Viro authored
    We can get false negative from __lookup_mnt() if an unrelated vfsmount
    gets moved.  In that case legitimize_mnt() is guaranteed to fail,
    and we will fall back to non-RCU walk... unless we end up running
    into a hard error on a filesystem object we wouldn't have reached
    if not for that false negative.  IOW, delaying that check until
    the end of pathname resolution is wrong - we should recheck right
    after we attempt to cross the mountpoint.  We don't need to recheck
    unless we see d_mountpoint() being true - in that case even if
    we have just raced with mount/umount, we can simply go on as if
    we'd come at the moment when the sucker wasn't a mountpoint; if we
    run into a hard error as the result, it was a legitimate outcome.
    __lookup_mnt() returning NULL is different in that respect, since
    it might've happened due to operation on completely unrelated
    Signed-off-by: default avatarAl Viro <>