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

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Message-ID: <m1skr7hphh.fsf@frodo.ebiederm.org>
Date:	Tue, 07 Oct 2008 17:58:34 -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:

> 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?

So in next_tid which is essentially what we would be doing.
I grab the rcu_lock.  Check something to see if the task I have
is still on the list, if it is then I know the next is valid until
the end of the rcu grace period.  Then I follow the next pointer,
and grab the lock again.

rcu is pain to get right but at least it is localized pain.

>> 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.  :-)

I don't think we can make it work but I think we need to exhaust that
avenue before saying that the VFS has to change.

Eric
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