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

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Message-ID: <m1k5cklzpc.fsf@frodo.ebiederm.org>
Date:	Tue, 07 Oct 2008 17:04:15 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Tejun Heo <tj@...nel.org>
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

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).

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.

Eric
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