linux-kernel - Re: [Xen-devel] [PATCH 3/3] Xen physical cpus interface

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Date:	Fri, 11 May 2012 15:00:41 -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 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.

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