linux-kernel - Re: [RFC][PATCH v2 08/11] sysfs: introduce new interface sysfs_get

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Date:	Wed, 19 May 2010 10:55:17 +0800
From:	Lin Ming <ming.m.lin@...el.com>
To:	Greg KH <greg@...ah.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	"eranian@...il.com" <eranian@...il.com>,
	"Gary.Mohr@...l.com" <Gary.Mohr@...l.com>,
	"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Paul Mackerras <paulus@...ba.org>,
	"David S. Miller" <davem@...emloft.net>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Will Deacon <will.deacon@....com>,
	Maynard Johnson <mpjohn@...ibm.com>,
	Carl Love <carll@...ibm.com>,
	Kay Sievers <kay.sievers@...y.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH v2 08/11] sysfs: introduce new interface
 sysfs_get_kobject

On Wed, 2010-05-19 at 04:08 +0800, Greg KH wrote:
> On Wed, May 19, 2010 at 01:48:43AM +0000, Lin Ming wrote:
> > Need this interface in the later sysfs pmu lookup.
> > 
> > struct kobject *sysfs_get_kobject(struct file *file);
> > Return the relative kobject of the sysfs file.
> 
> Ick, no.  Why would you ever have the file, yet not have the kobject
> already?  Something is really wrong if this is needed.  Or strange.  Or
> maybe both :)

Let me show you the scenario.

/sys/devices/system/cpu/event_source/
`-- id

/sys/devices/system/cpu/events/
|-- L1-dcache-load-misses
|   |-- event_source -> ../../event_source

$perf top -e L1-dcache-load-misses

Lookup the pmu used to handle L1-dcache-load-misses as below,

1. pmu_sys_fd =
open("/sys/devices/system/cpu/events/L1-dcache-load-misses/event_source/id", ...)

2. pmu_sys_file = fget_light(pmu_sys_fd, ....)

3. pmu_kobject = sysfs_get_kobject(pmu_sys_file)

4. pmu_kobject is embedded in struct pmu, pmu = container_of(kobj,
struct pmu, kobj);

> 
> thanks,
> 
> greg k-h

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