linux-kernel - Re: [ANNOUNCE] Status of unlocked

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Date:	Wed, 27 Oct 2010 13:16:36 -0500
From:	James Bottomley <James.Bottomley@...e.de>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Mike Anderson <andmike@...ux.vnet.ibm.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	Vasu Dev <vasu.dev@...ux.intel.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Mike Christie <michaelc@...wisc.edu>,
	Jens Axboe <jaxboe@...ionio.com>,
	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>,
	Jon Hawley <warthog9@...nel.org>,
	Brian King <brking@...ux.vnet.ibm.com>,
	Christof Schmitt <christof.schmitt@...ibm.com>,
	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [ANNOUNCE] Status of unlocked_qcmds=1 operation for .37

On Wed, 2010-10-27 at 11:06 -0700, Nicholas A. Bellinger wrote:
> On Wed, 2010-10-27 at 09:27 -0500, James Bottomley wrote:
> > On Wed, 2010-10-27 at 09:53 +0200, Andi Kleen wrote:
> > > > This sounds like a pretty reasonable compromise that I think is slightly
> > > > less risky for the LLDs with the ghosts and cob-webs hanging off of
> > > > them.
> > > 
> > > They won't get tested either next release cycle. Essentially
> > > near nobody uses them.
> > > 
> > > > 
> > > > What do you think..?
> > > 
> > > Standard linux practice is to simply push the locks down. That's a pretty
> > > mechanical operation and shouldn't be too risky
> > > 
> > > With some luck you could even do it with coccinelle.
> > 
> > Precisely ... if we can do the push down now as a mechanical
> > transformation we can put it in the current merge window as a low risk
> > API change.
> 
> I disagree that touching every single legacy LLD's SHT->queuecommand()
> and failure paths in that code is a low rist change.

It can be done mechanically.

> >   This gives us optimal exposure to the rc sequence to sort
> > out any problems that arise (or drivers that got missed) with the lowest
> > risk of such problems actually arising.
> 
> Yes, 
> 
> > Given the corner cases and the
> > late arrival of fixes, the serial number changes are just too risky for
> > the current merge window.
> 
> I think with andmike's testing and ACKs for the necessary scsi_error.c
> changes this would be an acceptable risk.

I already said why I didn't like this change.  Without the serial
number, there's no problem.

> > Having an API that changes depending on a
> > flag is also a high risk process because it's prone to further sources
> > of error.
> > 
> 
> I think this would be considered high risk if the setting of the flag
> explictly was required to obtain the default legacy operation.  With
> this series that is not the case, as the default SHT->unlocked_qcmd=0
> will allow legacy LLDs to function exactly the manner they expect, while
> allowing modern LLDs to run in host_lock-less mode.

Having a variable API based on a flag elsewhere is always a bad idea.

James


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