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

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Message-Id: <1288202802.5169.202.camel@haakon2.linux-iscsi.org>
Date:	Wed, 27 Oct 2010 11:06:42 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	James Bottomley <James.Bottomley@...e.de>
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 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.

>   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.

> 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.

Best,

--nab

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