linux-kernel - Re: [PATCH 1/8] scsi: Drop struct Scsi_Host->host

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Date:	Thu, 16 Sep 2010 20:02:32 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	James Bottomley <James.Bottomley@...e.de>
Cc:	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Vasu Dev <vasu.dev@...ux.intel.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Mike Christie <michaelc@...wisc.edu>,
	James Smart <james.smart@...lex.com>,
	Andrew Vasquez <andrew.vasquez@...gic.com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Hannes Reinecke <hare@...e.de>,
	Joe Eykholt <jeykholt@...co.com>,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 1/8] scsi: Drop struct Scsi_Host->host_lock around
	SHT->queuecommand()

On Thu, 2010-09-16 at 22:46 -0400, James Bottomley wrote:
> On Thu, 2010-09-16 at 15:35 -0700, Nicholas A. Bellinger wrote:
> > From: Nicholas Bellinger <nab@...ux-iscsi.org>
> > 
> > This patch converts scsi_dispatch_cmd() to only hold struct Scsi_Host->host_lock
> > for scsi_cmd_get_serial(), and drops the lock because the call into
> > the LLD with host->hostt->queuecommand().
> > 
> > Signed-off-by: Nicholas A. Bellinger <nab@...ux-iscsi.org>
> > ---
> >  drivers/scsi/scsi.c |    3 ++-
> >  1 files changed, 2 insertions(+), 1 deletions(-)
> > 
> > diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
> > index ad0ed21..06e5b3a 100644
> > --- a/drivers/scsi/scsi.c
> > +++ b/drivers/scsi/scsi.c
> > @@ -745,6 +745,7 @@ int scsi_dispatch_cmd(struct scsi_cmnd *cmd)
> >  	 * TODO: kill serial or move to blk layer
> >  	 */
> >  	scsi_cmd_get_serial(host, cmd); 
> > +	spin_unlock_irqrestore(host->host_lock, flags);
> 
> So at least from where I stand, my object is to reduce the number of
> times we take and release the lock, which this doesn't do.  As I said
> before: we need to figure out the rest, which likely includes an atomic
> for the serial number (which is almost unused).  I think the check
> against SHOST_DEL is fine unlocked.
> 

Ok, this makes sense if the shost_state == SHOST_DEL check really is OK
to be done w/o host_lock held (I really don't know on this one..)

So if that is the case, then Tim and Vasu can you guys have a look at
dropping the host_lock around serial number usage and post this for
review..?  I will plan to include the host_lock'less serial number patch
into lio-core-2.6.git/drop-host_lock with the rest of the LLD driver
conversion patches from this afternoon and respin a new branch for James
and Co. to review.

Thanks!

--nab


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