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

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Message-Id: <1288297634.5169.302.camel@haakon2.linux-iscsi.org>
Date:	Thu, 28 Oct 2010 13:27:14 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Boaz Harrosh <bharrosh@...asas.com>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	James Bottomley <James.Bottomley@...e.de>,
	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>, julia@...u.dk
Subject: Re: [ANNOUNCE] Status of unlocked_qcmds=1 operation for .37

On Thu, 2010-10-28 at 13:18 +0200, Boaz Harrosh wrote:
> On 10/28/2010 11:10 AM, Andi Kleen wrote:
> > On Wed, Oct 27, 2010 at 09:27:38AM -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.  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.  Given the corner cases and the
> >> late arrival of fixes, the serial number changes are just too risky for
> >> the current merge window.  Having an API that changes depending on a
> >> flag is also a high risk process because it's prone to further sources
> >> of error.
> > 
> > Here's a coccinelle script I came up with that does the push down.
> > It still adds a bogus empty line in front of the irqflags declaration
> > which I haven't managed to avoid yet. Other than the it seems
> > to DTRT on the SCSI drivers I tried.
> > 
> > -Andi
> > 
> > 
> > @ rule1 @
> > struct scsi_host_template t;
> > identifier qc;
> > @@
> > t.queuecommand = qc;
> > 
> > @ rule2 @
> > identifier rule1.qc;
> > identifier cmnd;
> > expression E;
> > statement S, S2;
> > @@
> > int qc(struct scsi_cmnd *cmnd, ...) 
> > {
> > 	... when != S
> > +	unsigned long irqflags;
> > 
> > +	spin_lock_irqsave(&cmnd->device->host->hostlock, irqflags);
> > 	S2
> > 	...
> > +	spin_unlock_irqrestore(&cmnd->device->host->hostlock, irqflags);
> > 	return E;
> > }
> > 
> 
> I disagree with your approach this introduces a spin_unlock_irqrestore
> call site at every return, in the usually huge queuecommand.
> 
> I'd say just do:
> - Rename XXX_queuecommand => __XXX_queuecommand_unlocked
> - Define new XXX_queuecommand
> 
> int qc(struct scsi_cmnd *cmnd, ...) 
> {
> 	unsigned long irqflags;
> 	int ret;
> 
> 	spin_lock_irqsave(&cmnd->device->host->hostlock, irqflags);
> 	ret = __XXX_queuecommand_unlocked(cmnd, ...)
> 	spin_unlock_irqrestore(&cmnd->device->host->hostlock, irqflags);
> 	return ret;
> }
> 
> Then when the driver is manually converted the __queuecommand_unlocked
> can be set into the scsi_host_template and the added function can
> be dropped.
> 

I would have to agree that approach does make a bit more sense..  Now
can some brave soul (/me looks at ak) code another script to automate
this for the identified legacy LLDs cases that need push down..?

8-)

--nab


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