Possibly wrong BIO usage in ide_multwrite

[Christophe Saout <christophe@saout.de>]

Hi!

I was just investigating where bio->bi_idx gets modified in the kernel.

I found these lines in ide-disk.c in ide_multwrite (DMA off, TASKFILE_IO
off):

> if (++bio->bi_idx >= bio->bi_vcnt) {
>     bio->bi_idx = 0;
>     bio = bio->bi_next;
> }

(rq->bio also gets changed but it's protected by the scratch buffer)

I think changing the bi_idx here is dangerous because
end_that_request_first needs this value to be unchanged because it
tracks the progress of the bio processing and updates bi_idx itself.

And bio->bi_idx = 0 is probably wrong because the bio can be submitted
with bio->bi_idx > 0 (if the bio was splitted and there are clones that
share the bio_vec array, like raid or device-mapper code).

If it really needs to play with bi_idx itself care should be taken to
reset bi_idx to the original value, not to zero.

I wasn't able to trigger a problem though, I don't know why exactly,
perhaps there are paths in __end_that_request_first that are not
interested in bi_dx. I still think there is something wrong with it.