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

[Bart Van Assche <bvanassche@acm.org>]

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? 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.

Bart.