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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [2.6.33-rc5] starting emacs makes lockdep warning
Cc: kosaki.motohiro@jp.fujitsu.com, Am??rico Wang <xiyou.wangcong@gmail.com>,
       "Eric W. Biederman" <ebiederm@xmission.com>,
       Tavis Ormandy <taviso@google.com>, Jeff Dike <jdike@addtoit.com>,
       Julien Tinnes <jln@google.com>, Matt Mackall <mpm@selenic.com>,
       LKML <linux-kernel@vger.kernel.org>, Oleg Nesterov <oleg@redhat.com>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>
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Date: Tue, 26 Jan 2010 16:45:41 +0900 (JST)
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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