linux-kernel - Re: lockdep trace from posix timers

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Message-ID: <20120820145550.GA18499@redhat.com>
Date:	Mon, 20 Aug 2012 16:55:50 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	rostedt <rostedt@...dmis.org>, dhowells <dhowells@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: lockdep trace from posix timers

On 08/20, Peter Zijlstra wrote:
>
> On Fri, 2012-08-17 at 17:14 +0200, Oleg Nesterov wrote:
> > I still think that task_work_add() should synhronize with exit_task_work()
> > itself and fail if necessary. But I wasn't able to convince Al ;)
>
> I'm not at all sure how that relates to needing task_lock() in the
> keyctl stuff.

To serialize with exit_mm() which clears ->mm.

We shouldn't do task_work_add(task) if the exiting task has already
passed exit_task_work(). There is no way to do this after ed3e694d7
(and I think this is wrong), so keyctl relies on the fact that
exit_task_work() is called after exit_mm().

> Also, can't task_work use llist stuff? That would also avoid using
> ->pi_lock.

Not sure.... task_work_add/run can use cmpxchg, but _cancel can't.

Oleg.

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