sorting out the freeze / quiesce mess

[Ming Lei]

On Wed, Nov 10, 2021 at 05:29:26PM +0800, Ming Lei wrote:
> On Wed, Nov 10, 2021 at 10:14:07AM +0100, 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?
> 
> freeze can make sure that all requests are done, quiesce can make sure that
> dispatch critical area(covered by hctx lock/unlock) is done.

Another difference: quiesce usually is used to stop to queue requests to
LLD, and driver needs no requets queued any more after the interface returns,
freeze can't do that.

Thanks,
Ming