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

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Date:	Tue, 26 Jan 2010 16:45:57 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	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

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.