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?
--
Jens Axboe
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