Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU and generation based scheme

[Bart Van Assche]

On Thu, 2017-12-14 at 11:19 -0800, tj@xxxxxxxxxx wrote:
> On Thu, Dec 14, 2017 at 06:51:11PM +0000, Bart Van Assche wrote:
> > On Tue, 2017-12-12 at 11:01 -0800, Tejun Heo wrote:
> > > --- a/block/blk-core.c
> > > +++ b/block/blk-core.c
> > > @@ -126,6 +126,8 @@ void blk_rq_init(struct request_queue *q, struct request *rq)
> > >  	rq->start_time = jiffies;
> > >  	set_start_time_ns(rq);
> > >  	rq->part = NULL;
> > > +	seqcount_init(&rq->gstate_seq);
> > > +	u64_stats_init(&rq->aborted_gstate_sync);
> > >  }
> > >  EXPORT_SYMBOL(blk_rq_init);
> > 
> > Sorry but the above change looks ugly to me. My understanding is that 
> > blk_rq_init() is only used inside the block layer to initialize legacy block
> > layer requests while gstate_seq and aborted_gstate_sync are only relevant
> > for blk-mq requests. Wouldn't it be better to avoid that blk_rq_init() is
> > called for blk-mq requests such that the above change can be left out? The
> > only callers outside the block layer core of blk_rq_init() I know of are
> > ide_prep_sense() and scsi_ioctl_reset(). I can help with converting the SCSI
> > code if you want.
> 
> This is also used by flush path.  We probably should clean that up,
> but let's worry about that later cuz flush handling has enough of its
> own complications.

We may have a different opinion about this but I think it is more than a
detail. This patch needs gstate_seq and aborted_gstate_sync to be preserved
across request state transitions from MQ_RQ_IN_FLIGHT to MR_RQ_IDLE.
blk_mq_init_request() is called at request allocation time so it's the right
context to initialize gstate_seq and aborted_gstate_sync. blk_rq_init()
however is called before a every use of a request. Sorry but I'm not
enthusiast about the code in blk_rq_init() that reinitializes state
information that should survive request reuse.

Bart.