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Date: Fri, 17 Aug 2012 17:14:47 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Jones <davej@redhat.com>, Linux Kernel <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, rostedt <rostedt@goodmis.org>,
        dhowells <dhowells@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: lockdep trace from posix timers
Message-ID: <20120817151447.GA7918@redhat.com>
References: <20120724203613.GA9637@redhat.com> <1345140478.29668.54.camel@twins>
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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.