linux-kernel - Re: [2.6.33-rc5] starting emacs makes lockdep warning

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Date:	Tue, 26 Jan 2010 01:14:15 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Américo Wang <xiyou.wangcong@...il.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Tavis Ormandy <taviso@...gle.com>,
	Jeff Dike <jdike@...toit.com>, Julien Tinnes <jln@...gle.com>,
	Matt Mackall <mpm@...enic.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [2.6.33-rc5] starting emacs makes lockdep warning

Américo Wang <xiyou.wangcong@...il.com> writes:

> On Tue, Jan 26, 2010 at 3:45 PM, KOSAKI Motohiro
> <kosaki.motohiro@...fujitsu.com> wrote:
>> Hi
>>
>>> On Tue, Jan 26, 2010 at 02:01:12PM +0800, Am??rico Wang wrote:
>>>
>>> > I agree, it seems that patch is useless, since we already
>>> > do lock_kernel() before calling __f_setown()...
>>>
>>> What's to prevent pid from being freed under us?  BKL won't...
>>
>> I don't understand this issue at all. so, this is stupid dumb question.
>> Why can't we write following code?
>>
>>
>>                enum pid_type type;
>>                struct pid *pid;
>>                if (!waitqueue_active(&tty->read_wait))
>>                        tty->minimum_to_wake = 1;
>>                spin_lock_irqsave(&tty->ctrl_lock, flags);
>>                if (tty->pgrp) {
>>                        pid = tty->pgrp;
>>                        type = PIDTYPE_PGID;
>>                } else {
>>                        pid = task_pid(current);
>>                        type = PIDTYPE_PID;
>>                }
>>                get_pid(pid)                                    // insert here
>>                spin_unlock_irqrestore(&tty->ctrl_lock, flags);
>>                retval = __f_setown(filp, pid, type, 0);
>>                put_pid(pid)                                    // insert here
>>
>
> Yeah, this seems reasonable for me, but not sure if this is the best fix.

That or tweak __f_setown to use irqsave/irqrestore variants for it's
locks, __f_setown is already atomic.  I prefer that direction because the
code is just a little simpler.

Eric