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    Subject[PATCH 2/4] unionfs: 32-bit needs lock for i_size
    LTP's iogen01 doio tests hang nicely on 32-bit SMP when /tmp is a unionfs
    mount of a tmpfs. See the comment on i_size_write in linux/fs.h: it needs
    to be locked, otherwise i_size_read can spin forever waiting for a lost
    seqcount update.

    Most filesystems are already holding i_mutex for this, but unionfs calls
    fsstack_copy_inode_size from many places, not necessarily holding i_mutex.
    Use the low-level i_lock within fsstack_copy_inode_size when 32-bit SMP.

    Signed-off-by: Hugh Dickins <>
    This works for me, to help get through LTP, but is not enough: what about
    those places which use i_size_write in common code called by unionfs?
    They ought to participate in this i_locking too. Needs an audit to
    enumerate such places and decide what course to take.

    Bigger question: is unionfs calling down to filesystems without holding
    their i_mutex, where some may be relying on their i_mutex to be held?

    fs/stack.c | 6 ++++++
    1 file changed, 6 insertions(+)

    --- 2.6.24-rc5-mm1/fs/stack.c 2007-12-05 10:38:38.000000000 +0000
    +++ unionfs2/fs/stack.c 2007-12-05 16:50:15.000000000 +0000
    @@ -21,8 +21,14 @@
    void fsstack_copy_inode_size(struct inode *dst, const struct inode *src)
    +#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
    + spin_lock(&dst->i_lock);
    i_size_write(dst, i_size_read((struct inode *)src));
    dst->i_blocks = src->i_blocks;
    +#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
    + spin_unlock(&dst->i_lock);

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      Last update: 2007-12-18 23:17    [W:0.023 / U:9.792 seconds]
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