Re: [RFC][PATCH v2 08/11] sysfs: introduce new interfacesysfs_get_kobject

[Lin Ming]

On Wed, 2010-05-19 at 11:35 +0800, Greg KH wrote:
> On Wed, May 19, 2010 at 10:55:17AM +0800, Lin Ming wrote:
> > 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", ...)
> 
> You do that within the kernel?  or from userspace?

>From userspace.
See "[RFC][PATCH v2 11/11] perf top: demo of how to use the sysfs
interfac" for a simple example.

> 
> Either way, your id show callback will get called when you read or write
> to that file, right?  Then you have your kobject.
> 
> > 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);
> 
> Oh no.
> 
> You are just using the kobject sysfs tree to store your kobjects so you
> can look them up again some time in the future from within the kernel?

Yes.

> Seriously?
> 
> What's wrong with a simple list of kobjects?  Or what the rest of the
> kernel does (busses and devices and iterating over the devices for a
> bus)?

A simple list of pmus is also added in this patch series

There are 2 pmu lookup methods in this patch series.
1. Search a simple list of pmus
2. Lookup pmu via sysfs 

Method 1 is used for back compatibility.

> 
> Don't act like userspace here and try to use the sysfs filesystem layout
> as a lookup into the kobject you are trying to find.  That's a
> horrible abuse of sysfs.  One of the worse I have ever seen.  And I've
> seen a lot of sysfs abuse over the years...

Ah, it's that bad... I need to re-think how to export pmus/events to
userspace...

Lin Ming

> 
> ick.
> 
> greg k-h

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