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    SubjectRe: Linux 2.4.19-pre3
    On Tue, Mar 12 2002, Marcelo Tosatti wrote:
    > So, Jens, could you please explain the problem in the interrupt handlers
    > in detail ?

    Ok... It affects all the pio handlers in ide-taskfile.c,
    multi-write/read as well. The address for pio transfers is calculated
    like so:

    va = rq->buffer + (rq->nr_sectors - rq->current_nr_sectors) * SECTOR_SIZE;

    which is wrong for two reasons. First of all, rq->buffer cannot be
    indexed for the entire nr_sectors range -- it's per definition only the
    first segment in the request, and can as such only be indexed within the
    first current_nr_sectors number of sectors. The above can be grossly out
    of range... Second, nr_sectors and current_nr_sectors are indexing two
    different things -- the former indexes the entire request (all segments)
    while the latter indexes only the first segments. So

    foo = rq->nr_sectors - rq->current_nr_sectors;

    makes no sense _at all_ and can only be wrong.

    So why does 2.4.19-pre3 work for pio at all? For the same reason that
    Andre never found this problem in 2.5 either: the taskfile interrupt
    handlers are _never_ used in pio mode. In 2.5 it was by accident, and
    when the merge happened they did indeed get used. It ate disks, very
    quickly. Take a look at drivers/ide/ide-disk.c, line 64:

    # undef __TASKFILE__IO /* define __TASKFILE__IO */
    #else /* CONFIG_IDE_TASKFILE_IO */
    # undef __TASKFILE__IO
    #endif /* CONFIG_IDE_TASKFILE_IO */

    It's a mess... This really should have been fixed prior to 2.4
    inclusion. Oh well.

    Jens Axboe

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