Re: [PATCH] blk-mq: Properly init bios from blk_mq_alloc_request_hctx()

[John Garry]

On 25/10/2022 10:00, Ming Lei wrote:
> > My use case is in SCSI EH domain. For my HBA controller of interest, to
> > abort an erroneous IO we must send a controller proprietary abort
> > command on same HW queue as original command. So we would need to
> > allocate this abort request for a specific HW queue.
> IMO, it is one bad hw/sw interface.
>
> First such request has to be reserved, since all inflight IOs can be in error.

Right

> Second error handling needs to provide forward-progress, and it is supposed
> to not require external dependency, otherwise easy to cause deadlock, but
> here request from specific HW queue just depends on this queue's cpumask.
>
> Also if it has to be reserved, it can be done as one device/driver private
> command, so why bother blk-mq for this special use case?

I have a series for reserved request support, which I will send later. 
Please have a look. And as I mentioned, I would prob not end up using 
blk_mq_alloc_request_hctx() anyway.

> > I mentioned before that if no hctx->cpumask is online then we don't need
> > to allocate a request. That is because if no hctx->cpumask is online,
> > this means that original erroneous IO must be completed due to nature of
> > how blk-mq cpu hotplug handler works, i.e. drained, and then we don't
> > actually need to abort it any longer, so ok to not get a request.
> No, it is really not OK, if all cpus in hctx->cpumask are offline, you
> can't allocate
> request on the specified hw queue, then the erroneous IO can't be handled,
> then cpu hotplug handler may hang for ever.

If the erroneous IO is still in-flight from blk-mq perspective, then how 
can hctx->cpumask still be offline? I thought that we guarantee that 
hctx->cpumask cannot go offline until drained.

Thanks,
John