linux-kernel - Re: sysfs: tagged directories not merged completely yet

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Date:	Wed, 08 Oct 2008 09:20:35 +0900
From:	Tejun Heo <tj@...nel.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Al Viro <viro@...IV.linux.org.uk>,
	Benjamin Thery <benjamin.thery@...l.net>,
	Greg KH <greg@...ah.com>, linux-kernel@...r.kernel.org,
	"Serge E. Hallyn" <serue@...ibm.com>,
	Al Viro <viro@....linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: sysfs: tagged directories not merged completely yet

Hello,

Eric W. Biederman wrote:
> Tejun Heo <tj@...nel.org> writes:
> 
>> Hello, a bit of additions after some sleep.
> 
>> Heh... it did sound like a plan but I don't think the plan would solve
>> the problem.  filldir can't be put in rcu read critical section.  :-p
> 
> There is srcu and there is the trick of grabbing the reference count
> on the current sysfs_dirent over the filldir and dropping the rcu
> lock (which works for proc).

Hmmm... I'm probably missing something (and being lazy) but how does it
guarantee the validity of the next pointer after dropping the rcu lock?

> To cut down on lock overhead from user space accesses that works.
> 
>>>> The revalidate on access model doesn't appear to have a way to track
>>>> remote renames.  Something sysfs supports.
>>> Yeap, IIRC, one of the reasons why sysfs wasn't converted over to
>>> sysfs was because sysfs guarantees inode doesn't change over rename or
>>> move so that notifications keep working over renames.
>> s/over to sysfs/over to revalidation/ and s/inode/dentry/.  Maybe we can
>> just ignore dnotify?  :-(
> 
> Well there are more cases than dnotify, there is the renaming of directories
> in sysfs, although rare that I think get awkward if we use revalidation.
> 
> I'm still not certain how we can get the lock ordering so it doesn't
> cause us problems.  I will look at revalidation and what the other
> distributed filesystems are doing and see if that might work.  If it
> doesn't we need refactor the VFS locking.

Yeah, if we can make sysfs behave like other distributed filesystems, it
would be great.  :-)

Thanks.

-- 
tejun
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