linux-kernel - Re: [PATCH 0/4] Was: deferring _

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Date:	Sat, 30 Jun 2012 19:41:33 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	". James Morris" <jmorris@...ei.org>,
	linux-security-module@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 0/4] Was: deferring __fput()

On 06/30, Al Viro wrote:
>
> On Thu, Jun 28, 2012 at 06:45:39PM +0200, Oleg Nesterov wrote:
> > Forgot to mention...
> >
> > And I still think that task_work_add() should not succeed unconditionally,
> > it synchronize with exit_task_work(). Otherwise keyctl_session_to_parent()
> > is broken.
>
> Hmm...  Look: if nothing else, we have
>         /* the parent mustn't be init and mustn't be a kernel thread */
>         if (parent->pid <= 1 || !parent->mm)
>                 goto unlock;
> in the caller.  OTOH, on the exit side we have exit_mm() done first.  And
> that will have ->mm set to NULL.  So we are closing a very narrow race to start
> with.  So why not do the following and be done with that?

Of course we can fix keyctl_session_to_parent(). But why? I mean, why
do you dislike the idea to put this synchronization into add/run ?

IMO, this makes task_work much less useful/convenient. Every caller
has to fight with this race somehow. And the lock it can take depends
on the context, say, you can't use task_lock() in irq.

IOW, what is wrong with

	[PATCH 2/4] task_work: don't rely on PF_EXITING
	http://marc.info/?l=linux-kernel&m=134082265321691

and
	[PATCH 3/4] task_work: deal with task_work callbacks adding more work
	http://marc.info/?t=134082275400004

?

perhaps you do not like the fact that the exiting task takes pi_lock
unconditionally?



and in fact I think that probably it makes sense to change fput,

	- task_work_add(current, ...);
	+ BUG_ON(task_work_add(current, ...) != 0);

Oleg.

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