sorting out the freeze / quiesce mess

[Ming Lei]

On Wed, Nov 10, 2021 at 11:22:05AM +0100, Hannes Reinecke wrote:
> On 11/10/21 10:14 AM, Christoph Hellwig wrote:
> > Hi Jens and Ming,
> > 
> > I've been looking into properly supporting queue freezing for bio based
> > drivers (that is only release q_usage_counter on bio completion for them).
> > And the deeper I look into the code the more I'm confused by us having
> > the blk_mq_quiesce* interface in addition to blk_freeze_queue.  What
> > is a good reason to do a quiesce separately from a freeze?
> > 
> IIRC the 'quiesce' interface was an abstraction from the SCSI 'quiesce'
> operation, where we had to stop all I/O except for TMFs and scanning.
> And 'freeze' was designed fro stopping all I/O.
> 
> But I'm not sure if that ever was the distinction, or if it still
> applies today.
> 
> And yeah, I've been wondering myself.
> 
> Probably we should just kill the 'quiesce' stuff and see where we end up :-)

In case of EH, no queued requests can be completed, however driver still
needs to stop queue and reset hardware, then how can you use freeze to stop
queue? See nvme_dev_disable().

Freeze can stop to allocate new request and drain all queued requests, but
it can't prevent IO from being queued to LLD. On the contrary,
blk_mq_freeze_queue_wait() requires LLD to handle IO for moving on,
otherwise it will wait forever.

Thanks,
Ming