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

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Date:	Wed, 16 Jul 2008 14:09:55 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Tejun Heo <htejun@...il.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.

Tejun Heo <htejun@...il.com> writes:
>> 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.

The vfs still does. So at least for directory tree manipulation  we
need to hold i_mutex before we grab sysfs_mutex.

I think that means we need to unscramble the whole set of locking
order issues.

In lookup we have:
local_vfs_lock -> fs_global_lock

In modifications we have:
fs_global_lock -> local_vfs_lock

Which is the definition of a lock ordering problem.

Currently we play jump through some significant hoops to keep things
in local_vfs_lock -> fs_global_lock order.

If we also take the rename_mutex on directory adds and deletes we
may be able to keep jumping through those hoops.  However I expect
we would be in a much better situation if we could figure out how
to avoid the problem.

It looks like the easy way to handle this is to make the sysfs_dirent
list rcu protected.  Which means we can fix our lock ordering problem
without VFS modifications.  Allowing the locking to always
be: sysfs_mutex ... i_mutex.

After that it would be safe and a good idea to have unshared
inodes between superblocks, just so we don't surprise anyone
making generic VFS assumptions.

Eric
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