RE: [PATCH V2] blk-mq: Set request mapping to NULL in blk_mq_put_driver_tag

[Kashyap Desai <kashyap.desai@broadcom.com>]

>
> I actually took a look at scsi_host_find_tag() - what I think needs fixing
> here is
> that it should not return a tag that isn't allocated.
> You're just looking up random stuff, that is a recipe for disaster.
> But even with that, there's no guarantee that the tag isn't going away.

Got your point. Let us fix in <mpt3sas> driver.

>
> The mpt3sas use case is crap. It's iterating every tag, just in case it
> needs to do
> something to it.

Many drivers in scsi layer is having similar trouble.  May be they are less
exposed. That was a main reason, I thought to provide common fix in block
layer.

>
> My suggestion would be to scrap that bad implementation and have
> something available for iterating busy tags instead. That'd be more
> appropriate and a lot more efficient that a random loop from 0..depth.
> If you are flushing running commands, looking up tags that aren't even
> active
> is silly and counterproductive.

We will address this issue through <mpt3sas> driver changes in two steps.
1. I can use  driver's internal memory and will not rely on request/scsi
command. Tag 0...depth loop  is not in main IO path, so what we need is
contention free access of the list. Having driver's internal memory and
array will provide that control.
2. As you suggested best way is to use busy tag iteration.  (only for blk-mq
stack)


Thanks for your feedback.

Kashyap

>
> --
> Jens Axboe