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

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Message-Id: <20100126164018.1D59.A69D9226@jp.fujitsu.com>
Date:	Tue, 26 Jan 2010 16:45:41 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	kosaki.motohiro@...fujitsu.com,
	Am??rico Wang <xiyou.wangcong@...il.com>,
	"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

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





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