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Date: Sat, 23 Jun 2012 22:11:39 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        ". James Morris" <jmorris@namei.org>,
        linux-security-module@vger.kernel.org,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: deferring __fput()
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On Sat, Jun 23, 2012 at 10:01:41PM +0100, Al Viro wrote:
> On Sat, Jun 23, 2012 at 10:38:00PM +0200, Oleg Nesterov wrote:
> 
> > > Matter of fact, it would become identical to struct rcu_head
> > > after that...
> > 
> > This is not clear to me... Why this is good?
> 
> Occam's Razor.
> 
> > I understand that sizeof(task_work) == sizeof(rcu_head) would be
> > nice, probably you meant just this?
> 
> More than that - the callback type is also the same (pointer to such
> struct -> void).  IOW, they both look like two instances of the
> same thing ("list of callbacks"), differing only in what and
> when does calling.

BTW, scratch that "task_work + cred" - cred has rcu_head in it.
So we can put a union in there and slap the trimmed task_work into 
it.  Voila - there goes separate allocation and *any* need for ->data.
It's just container_of(), in that case as well.