linux-kernel - Re: lockdep trace from posix timers

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Message-ID: <20120817151747.GA8248@redhat.com>
Date:	Fri, 17 Aug 2012 17:17:47 +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 08/17, Oleg Nesterov wrote:
>
> On 08/16, Peter Zijlstra wrote:
> >
> >      write_lock_irq(&tasklist_lock)
> >      task_lock(parent)		parent->alloc_lock
>
> And this is already wrong. See the comment above task_lock().
>
> > And since it_lock is IRQ-safe and alloc_lock isn't, you've got the IRQ
> > inversion deadlock reported.
>
> Yes. Or, IOW, write_lock(tasklist) is IRQ-safe and thus it can't nest
> with alloc_lock.
>
> > David, Al, anybody want to have a go at fixing this?
>
> 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 ;)

And this is my old patch: http://marc.info/?l=linux-kernel&m=134082268721700
It should be re-diffed of course.

Oleg.

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