netdev - Re: [PATCH 12/15] driver core: Implement tagged directory support for device classes.

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Date:	Wed, 16 Jul 2008 15:48:39 +0900
From:	Tejun Heo <htejun@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Greg KH <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Lezcano <dlezcano@...ibm.com>,
	linux-kernel@...r.kernel.org, Al Viro <viro@....linux.org.uk>,
	Linux Containers <containers@...ts.osdl.org>,
	Benjamin Thery <benjamin.thery@...l.net>,
	netdev@...r.kernel.org
Subject: Re: [PATCH 12/15] driver core: Implement tagged directory support
 for device classes.

Hello, Eric.

Eric W. Biederman wrote:
> Tejun Heo <htejun@...il.com> writes:
> 
>> It's a bit scary tho.  Working inode->i_dentry or dentry->d_alias
>> crosses multiple sb's.  sysfs isn't too greedy about dcache/icache.
>> Only open files and directories hold them and only single copy of
>> sysfs_dirent is there for most nodes.  Wouldn't it be better to stay on
>> the safer side and use separate inode hierarchy?
> 
> To do that I believe we would need to ensure sysfs does not use 
> the inode->i_mutex lock except to keep the VFS layer out.  Allowing us
> to safely change the directory structure, without holding it.

I don't think sysfs is depending on i_mutex anymore but I need to go
through the code to make sure.

> You raise a good point about inode->i_dentry and dentry->d_alias.
> Generally they are used by fat like filesystems but I am starting to
> see uses in generic pieces of code.  I don't see any problems today
> but yes it would be good to do the refactoring to allow us to duplicate
> the inodes. 

Yeah, I can't spot any place which can cause actual problem yet but it's
still scary as we're breaking a vfs assumption and even if it's not a
problem now, future seemingly unrelated changes can break things subtly.

Thanks.

-- 
tejun
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