On Thu, Sep 21 2006, Mike Christie wrote:
> Jens Axboe wrote:
> > On Thu, Sep 21 2006, Mike Christie wrote:
> >> James Smart wrote:
> >>> Why does your LLD need to reach up into the block layer to find an i/o ?
> >>>
> >>> Even if you are using tags as an index into something... I would think that
> >>> having to go up to the blk layer to retrieve a something that should have
> >>> already been known lower in the driver leaves room for race conditions.
> >>>
> >> This is how the blk and scsi tag api work for queue based tagging. We
> >> have the scsi_find_tag() function which takes a scsi device and than
> >> calls blk_queue_find_tag to get the request. It then does all the magic
> >> sdev to request_queue and request to scsi command work and pass the LLD
> >> the scsi command for the tag. So we can either add a driver array and do
> >> some tag to scsi command or driver stucture mapping or we can use the
> >> array already created in the scsi host block queue tag. And as you see
> >> Dave's patch did the latter in the spirit of not duplicating what
> >> scsi-ml or the block layer already do.
> >>
> >> I think there are some basic races though. For example, scsi_request_fn
> >> calls blk_queue_start_tag with only the queue lock held and so if the
> >> request_fn was called for two devices on the same host at the same time
> >> they both could call find_first_zero_bit on the shared bqt->tag_map and
> >> end up getting the same tag.
> >
> > Hrmpf good point, I suspect that would be easy enough to fix with just
> > using a
> >
> > do {
> > tag = ffz_bit(..);
> > } while (test_and_set_bit(tag, map);
> >
> > construct. Agree?
> >
>
> I think so. I think this is similar to what Dave was going to do too.
Already checked in such a fix:
http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=7ddbe6863ac4b7a5588aa4a70f262c098f3c469a
--
Jens Axboe
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