linux-kernel - RE: [PATCH 00/40] Staging: hv: Driver cleanup

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Message-ID: <6E21E5352C11B742B20C142EB499E0480816C4BF@TK5EX14MBXC126.redmond.corp.microsoft.com>
Date:	Fri, 1 Jul 2011 13:31:30 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	"gregkh@...e.de" <gregkh@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"virtualization@...ts.osdl.org" <virtualization@...ts.osdl.org>
Subject: RE: [PATCH 00/40] Staging: hv: Driver cleanup



> -----Original Message-----
> From: Christoph Hellwig [mailto:hch@...radead.org]
> Sent: Friday, July 01, 2011 4:22 AM
> To: KY Srinivasan
> Cc: Christoph Hellwig; gregkh@...e.de; linux-kernel@...r.kernel.org;
> devel@...uxdriverproject.org; virtualization@...ts.osdl.org
> Subject: Re: [PATCH 00/40] Staging: hv: Driver cleanup
> 
> On Thu, Jun 30, 2011 at 11:28:27PM +0000, KY Srinivasan wrote:
> > > On Wed, Jun 29, 2011 at 07:38:21AM -0700, K. Y. Srinivasan wrote:
> > > > Further cleanup of the hv drivers:
> > > >
> > > > 	1) Cleanup the reference counting mess for both stor and net devices.
> > >
> > > I really don't understand the need for reference counting on the storage
> > > side, especially now that you only have a SCSI driver.  The SCSI
> > > midlayer does proper counting on it's objects (Scsi_Host, scsi_device,
> > > scsi_cmnd), so you'll get that for free given that SCSI drivers just
> > > piggyback on the midlayer lifetime rules.
> >
> > The reference counting allows us to properly deal with messages coming back
> from the host
> > to the guest with a racing remove of the device. I am told these messages could
> potentially be
> > not a response to a message sent from the guest.
> 
> To deal with that the scsi subsystem has a two stage teardown for the
> SCSI host.  First you call scsi_remove_host, at which point no new I/O
> to it can be started.  After that you can wait for all outstanding
> messages, and finally you do a scsi_host_put to drop the long-term
> reference to it, and eventually free it.

That is what I have currently implemented both on the stor as well
as the net side with regards to the device specific objects hanging off of
struct hv_device (the ext field). If it is ok with you, I want to do what you
had suggested earlier. Keep the patches as they are with regards to 
reference counting for now. We can always simplify the code later.

Regards,

K. Y
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