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Date:   Fri, 8 Mar 2019 11:15:51 -0700
From:   Keith Busch <keith.busch@intel.com>
To:     Bart Van Assche <bvanassche@acm.org>, t@localhost.localdomain
Cc:     linux-nvme@lists.infradead.org, linux-block@vger.kernel.org,
        Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
        Sagi Grimberg <sagi@grimberg.me>
Subject: Re: [PATCH 1/5] blk-mq: Export reading mq request state
Message-ID: <20190308181551.GB5214@localhost.localdomain>
References: <20190308174006.5032-1-keith.busch@intel.com>
 <1552068443.45180.24.camel@acm.org>
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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.