Date:	Fri, 11 May 2012 16:48:24 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	"Liu, Jinsong" <jinsong.liu@...el.com>
Cc:	"'xen-devel@...ts.xensource.com'" <xen-devel@...ts.xensource.com>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 3/3] Xen physical cpus interface

On Fri, May 11, 2012 at 08:31:15PM +0000, Liu, Jinsong wrote:
> Konrad Rzeszutek Wilk wrote:
> > On Fri, May 11, 2012 at 06:04:21PM +0000, Liu, Jinsong wrote:
> >> Konrad Rzeszutek Wilk wrote:
> >>> On Fri, May 11, 2012 at 01:12:13PM +0000, Liu, Jinsong wrote:
> >>>> Liu, Jinsong wrote:
> >>>>> Just notice your reply (so quick :)
> >>>>> 
> >>>>> Agree and will update later, except 1 concern below.
> >>>>> 
> >>>>> Konrad Rzeszutek Wilk wrote:
> >>>>>>> 
> >>>>>>> Hmm, it's good if it's convenient to do it automatically via
> >>>>>>> dev->release. However, dev container (pcpu) would be free at
> >>>>>>> some other error cases, so I prefer do it 'manually'.
> >>>>>> 
> >>>>>> You could also call pcpu_release(..) to do it manually.
> >>>>>> 
> >>>>> 
> >>>>> that means kfree(pcpu) would be done twice at some error cases,
> >>>>> do you think it really good? 
> >>>>> 
> >>>> 
> >>>> Ping.
> >>>> 
> >>>> I think error recovery should be kept inside error logic level
> >>>> itself, if try to recover upper level error would bring trouble.
> >>>> 
> >>>> In our example, there are 2 logic levels:
> >>>> pcpu level (as container), and dev level (subfield used for sys)
> >>> 
> >>> So you need to untangle free_pcpu from doing both. Meaning one does
> >>> the SysFS and the other deals with free-ing the structure and
> >>> removing itself from the list.
> >>> 
> >> 
> >> free_cpu is very samll, just consist of the 2 parts your said:
> >> * pcpu_sys_remove() deal with sysfs
> >> * list_del/kfree(pcpu) deal with pcpu
> >> 
> >>> 
> >>>> dev->release should only recover error occurred at dev/sys level,
> >>>> and the pcpu error should be recovered at pcpu level.
> >>>> 
> >>>> If dev->release try to recover its container pcpu level error, like
> >>>> list_del/kfree(pcpu), it would make confusing. i.e., considering
> >>>> pcpu_sys_create(), 2 error cases: device_register fail, and
> >>>> device_create_file fail --> how can the caller decide kfree(pcpu)
> >>>> or not?
> >>> 
> >>> Then you should free it manually. But you can do this by a wrapper
> >>> function: 
> >>> 
> >>> __pcpu_release(..) {
> >>> 	..
> >>> 	/* Does the removing itself from the list and kfree the pcpu */ }
> >>> pcpu_release(..) {
> >>> 	struct pcpcu *p= container_of(..)
> >>> 	__pcpu_release(p);
> >>> }
> >>> 
> >>> dev->release = &pcpu_release;
> >>> 
> >> 
> >> Too weird way. If we want to release dev itself it's good to use
> >> dev->release, but for pcpu I doubt it. (consider the example I gave
> >> --> why we create issue (it maybe solved in weird method I agree),
> >> just for using dev->release?)  
> >> 
> >> In kernel many dev->release keep NULL.
> >> An example of using dev->release is cpu/mcheck/mce.c -->
> >> mce_device_release(), it *just* deal dev itself. 
> > 
> > OK? I am not sure what are we arguing here anymore?
> > I think using 'kfree(pcpu)' on the error paths (as long as it is
> > done before device_register) is OK. I think that seperating
> > the SysFS deletion from the pcpu deletion should be done to
> > avoid races. Perhaps the SysFS deletion function should also
> > remove the pcpu from the list.
> 
> How about static array pcpu[NR_CPUS]?
> It seems solve all issues we argued :)

Ugh. That could mean a structure of more than 4K of items.
Lets stick with making it dynamic.
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