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    SubjectRe: Possible race between direct IO and JBD?
    On Fri, 25 Apr 2008 16:38:23 -0700 Mingming Cao <cmm@us.ibm.com> wrote:

    > Hi,
    >
    > While looking at a bug related to direct IO returns to EIO, after
    > looking at the code, I found there is a window that
    > try_to_free_buffers() from direct IO could race with JBD, which holds
    > the reference to the data buffers before journal_commit_transaction()
    > ensures the data buffers has reached to the disk.
    >
    > A little more detail: to prepare for direct IO, generic_file_direct_IO()
    > calls invalidate_inode_pages2_range() to invalidate the pages in the
    > cache before performaning direct IO. invalidate_inode_pages2_range()
    > tries to free the buffers via try_to free_buffers(), but sometimes it
    > can't, due to the buffers is possible still on some transaction's
    > t_sync_datalist or t_locked_list waiting for
    > journal_commit_transaction() to process it.
    >
    > Currently Direct IO simply returns EIO if try_to_free_buffers() finds
    > the buffer is busy, as it has no clue that JBD is referencing it.
    >
    > Is this a known issue and expected behavior? Any thoughts?

    Something like that might be possible, although people used to test
    buffered-vs-direct fairly heavily.

    generic_file_direct_IO() will run
    filemap_write_and_wait()->filemap_fdatawrite() under i_mutex, and this
    should run commits, write back dirty pages, etc.

    There might remain races though, perhaps with page faults.


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