Re: [RFC] [PATCH] qla4xxx driver resubmission

[Mike Christie <michaelc@cs.wisc.edu>]

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.