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    Subject[patch] direct-io: bug fix in dio handling write error
    Date
    From: "Chen, Kenneth W" <kenneth.w.chen@intel.com>

    There is a bug in direct-io on propagating write error up to the
    higher I/O layer. When performing an async ODIRECT write to a
    block device, if a device error occurred (like media error or disk
    is pulled), the error code is only propagated from device driver
    to the DIO layer. The error code stops at finished_one_bio(). The
    aysnc write, however, is supposedly have a corresponding AIO event
    with appropriate return code (in this case -EIO). Application
    which waits on the async write event, will hang forever since such
    AIO event is lost forever (if such app did not use the timeout
    option in io_getevents call. Regardless, an AIO event is lost).

    The discovery of above bug leads to another discovery of potential
    race window with dio->result. The fundamental problem is that
    dio->result is overloaded with dual use: an indicator of fall back
    path for partial dio write, and an error indicator used in the I/O
    completion path. In the event of device error, the setting of -EIO
    to dio->result clashes with value used to track partial write that
    activates the fall back path.

    It was also pointed out that it is impossible to use dio->result to
    track partial write and at the same time to track error returned
    from device driver. Because direct_io_work can only determines whether
    it is a partial write at the end of io submission and in mid stream
    of those io submission, a return code could be coming back from the
    driver. Thus messing up all the subsequent logic.

    Proposed fix is to separating out error code returned by the IO
    completion path from partial IO submit tracking. A new variable is
    added to dio structure specifically to track io error returned in the
    completion path.


    Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
    Acked-by: Zach Brown <zach.brown@oracle.com>
    Acked-by: Suparna Bhattacharya <suparna@in.ibm.com>
    Cc: Badari Pulavarty <pbadari@us.ibm.com>
    ---

    Patch tested by pulling a scsi drive while running aiostress as well
    as running test code in http://developer.osdl.org/daniel/AIO/TESTS/

    Andrew - please merge this version. Thank you.


    --- ./fs/direct-io.c.orig 2006-01-02 19:21:10.000000000 -0800
    +++ ./fs/direct-io.c 2006-03-21 01:28:48.704475280 -0800
    @@ -129,6 +129,7 @@ struct dio {
    /* AIO related stuff */
    struct kiocb *iocb; /* kiocb */
    int is_async; /* is IO async ? */
    + int io_error; /* IO error in completion path */
    ssize_t result; /* IO result */
    };

    @@ -250,6 +251,10 @@ static void finished_one_bio(struct dio
    ((offset + transferred) > dio->i_size))
    transferred = dio->i_size - offset;

    + /* check for error in completion path */
    + if (dio->io_error)
    + transferred = dio->io_error;
    +
    dio_complete(dio, offset, transferred);

    /* Complete AIO later if falling back to buffered i/o */
    @@ -406,7 +411,7 @@ static int dio_bio_complete(struct dio *
    int page_no;

    if (!uptodate)
    - dio->result = -EIO;
    + dio->io_error = -EIO;

    if (dio->is_async && dio->rw == READ) {
    bio_check_pages_dirty(bio); /* transfers ownership */
    @@ -964,6 +969,7 @@ direct_io_worker(int rw, struct kiocb *i
    dio->next_block_for_io = -1;

    dio->page_errors = 0;
    + dio->io_error = 0;
    dio->result = 0;
    dio->iocb = iocb;
    dio->i_size = i_size_read(inode);


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