linux-kernel - Re: lockdep trace from posix timers

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Message-ID: <20120817151447.GA7918@redhat.com>
Date:	Fri, 17 Aug 2012 17:14:47 +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 08/16, 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 ;)

Oleg.

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