Re: [PATCH 1/5] blk-mq: Export reading mq request state

[Keith Busch <keith.busch@intel.com>]

On Fri, Mar 08, 2019 at 10:42:17AM -0800, Bart Van Assche wrote:
> On Fri, 2019-03-08 at 11:15 -0700, Keith Busch wrote:
> > On Fri, Mar 08, 2019 at 10:07:23AM -0800, Bart Van Assche wrote:
> > > On Fri, 2019-03-08 at 10:40 -0700, Keith Busch wrote:
> > > > Drivers may need to know the state of their requets.
> > > 
> > > Hi Keith,
> > > 
> > > What makes you think that drivers should be able to check the state of their
> > > requests? Please elaborate.
> > 
> > Patches 4 and 5 in this series.
> >  
> > > > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> > > > index faed9d9eb84c..db113aee48bb 100644
> > > > --- a/include/linux/blkdev.h
> > > > +++ b/include/linux/blkdev.h
> > > > @@ -241,6 +241,15 @@ struct request {
> > > >  	struct request *next_rq;
> > > >  };
> > > >  
> > > > +/**
> > > > + * blk_mq_rq_state() - read the current MQ_RQ_* state of a request
> > > > + * @rq: target request.
> > > > + */
> > > > +static inline enum mq_rq_state blk_mq_rq_state(struct request *rq)
> > > > +{
> > > > +	return READ_ONCE(rq->state);
> > > > +}
> > > 
> > > Please also explain how drivers can use this function without triggering a
> > > race condition with the code that modifies rq->state.
> > 
> > Either queisced or within a timeout handler that already locks the
> > request lifetime.
> 
> Hi Keith,
> 
> For future patch series submissions please include a cover letter. The two patch
> series that you posted today don't have a cover letter so I can only guess what
> the purpose of these two patch series is. Is the purpose of this patch series
> perhaps to speed up error handling? If so, why did you choose the approach of
> iterating over outstanding requests and telling the block layer to terminate
> these requests? 

Okay, good point. Will do.

> I think that the NVMe spec provides a more elegant mechanism,
> namely deleting the I/O submission queues. According to what I read in the
> 1.3c spec deleting an I/O submission queue forces an NVMe controller to post a 
> completion for every outstanding request. See also section 5.6 in the NVMe
> 1.3c spec.

That's actually not what it says. The controller may or may not post a
completion entry with a delete SQ command. The first behavior is defined
in the spec as "explicit" and the second as "implicit". For implicit,
we have to iterate inflight tags.