Re: Regarding posted scsi midlyaer patchsets

[Jens Axboe <axboe@suse.de>]

On Tue, Apr 19 2005, Jens Axboe wrote:
> On Tue, Apr 19 2005, James Bottomley wrote:
> > On Tue, 2005-04-19 at 14:34 +0200, Jens Axboe wrote:
> > > On Mon, Apr 18 2005, Tejun Heo wrote:
> > > >  And, James, regarding REQ_SOFTBARRIER, if the REQ_SOFTBARRIER thing can
> > > > be removed from SCSI midlayer, do you agree to change REQ_SPECIAL to
> > > > mean special requests?  If so, I have three proposals.
> > > > 
> > > >  * move REQ_SOFTBARRIER setting to right after the allocation of
> > > > scsi_cmnd in scsi_prep_fn().  This will be the only place where
> > > > REQ_SOFTBARRIER is used in SCSI midlayer, making it less pervasive.
> > > >  * Or, make another API which sets REQ_SOFTBARRIER on requeue.  maybe
> > > > blk_requeue_ordered_request()?
> > > >  * Or, make blk_insert_request() not set REQ_SPECIAL on requeue.  IMHO,
> > > > this is a bit too subtle.
> > > > 
> > > >  I like #1 or #2.  Jens, what do you think?  Do you agree to remove
> > > > requeue feature from blk_insert_request()?
> > > 
> > > #2 is the best, imho. We really want to maintain ordering on requeue
> > > always, marking it softbarrier automatically in the block layer means
> > > the io schedulers don't have to do anything specific to handle it.
> > 
> > This is my preference too.  In general, block is the only one that
> > should care what the REQ_SOFTBARRIER flag actually means.  SCSI only
> > cares that it submits a non mergeable request.
> > 
> > I'm happy to separate the meaning of REQ_SPECIAL from req->special.
> 
> Isn't it just duplicate information anyways? I mean, just clear
> ->special if it isn't valid anymore. Having a seperate flag to indicate
> this seems a little suboptimal. It made more sense when ->cmd was a
> integer being READ, WRITE, etc. But as a seperate state now it doesn't.

Oh, and this is only true of SCSI, btw. REQ_SPECIAL should not be seen
outside of driver code, its meaning is defined solely by the driver.
SCSI ties it to ->special, but that is not necessarily true for any
other driver.

-- 
Jens Axboe