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    ext4: Fix possible deadlock between ext4_truncate() and ext4_get_blocks() · 487caeef
    Jan Kara authored
    During truncate we are sometimes forced to start a new transaction as
    the amount of blocks to be journaled is both quite large and hard to
    predict. So far we restarted a transaction while holding i_data_sem
    and that violates lock ordering because i_data_sem ranks below a
    transaction start (and it can lead to a real deadlock with
    ext4_get_blocks() mapping blocks in some page while having a
    transaction open).
    We fix the problem by dropping the i_data_sem before restarting the
    transaction and acquire it afterwards. It's slightly subtle that this
    1) By the time ext4_truncate() is called, all the page cache for the
    truncated part of the file is dropped so get_block() should not be
    called on it (we only have to invalidate extent cache after we
    reacquire i_data_sem because some extent from not-truncated part could
    extend also into the part we are going to truncate).
    2) Writes, migrate or defrag hold i_mutex so they are stopped for all
    the time of the truncate.
    This bug has been found and analyzed by Theodore Tso <>.
    Signed-off-by: default avatarJan Kara <>
    Signed-off-by: default avatar"Theodore Ts'o" <>