linux-kernel - [PATCH 0/3] (Was: fput: task_work_add() can fail if the caller has passed exit_task

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Message-ID: <20130615172959.GA14656@redhat.com>
Date:	Sat, 15 Jun 2013 19:29:59 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Andrey Vagin <avagin@...nvz.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org, Huang Ying <ying.huang@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: [PATCH 0/3] (Was: fput: task_work_add() can fail if the caller has
	passed exit_task_work())

On 06/14, Andrew Morton wrote:
>
> On Fri, 14 Jun 2013 21:09:47 +0200 Oleg Nesterov <oleg@...hat.com> wrote:
>
> > +		if (likely(!in_interrupt() && !(task->flags & PF_KTHREAD))) {
> > +			init_task_work(&file->f_u.fu_rcuhead, ____fput);
> > +			if (!task_work_add(task, &file->f_u.fu_rcuhead, true))
> > +				return;
>
> A comment here would be useful, explaining the circumstances under
> which we fall through to the delayed fput.

Thanks!

> This is particularly needed
> because kernel/task_work.c is such undocumented crap.

It seems that you are trying to force me to make the doc patch ;)
OK, I'll try. task_work.c needs a couple of cosmetic cleanups anyway.

> > +		spin_lock_irqsave(&delayed_fput_lock, flags);
> > +		list_add(&file->f_u.fu_list, &delayed_fput_list);
> > +		schedule_work(&delayed_fput_work);
> > +		spin_unlock_irqrestore(&delayed_fput_lock, flags);
>
> OT: I don't think that schedule_work() needs to be inside the locked
> region.  Scalability improvements beckon!

Yeees, I thought about this too.

Performance-wise this can't really help, this case is unlikely. But
I think this change makes this code a bit simpler, so please see 1/3.

2/3 fixes the (theoretical) bug in llist_add() and imho cleanups the
code.

3/3 comes as a separate change because I do not want to argue if
someone dislike the non-inline llist_add(). But once again, we can
make llist_add_batch() inline, and I believe it is never good to
duplicate the code even if it is simple.

Oleg.

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