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

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Date:	Tue, 26 Jan 2010 04:33:38 -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 5:14 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> 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.
>>
>
> Oh, very good advice!
>
> Patch is below.
>
> -------------->
> Commit 703625118 causes a lockdep warning:
>
> [ INFO: possible irq lock inversion dependency detected ]
> 2.6.33-rc5 #77
> ---------------------------------------------------------
> emacs/1609 just changed the state of lock:
>  (&(&tty->ctrl_lock)->rlock){+.....}, at: [<ffffffff8127c648>]
> tty_fasync+0xe8/0x190
> but this lock took another, HARDIRQ-unsafe lock in the past:
>  (&(&sighand->siglock)->rlock){-.....}
>
> This is due to we use write_lock_irq() in __f_setown() which turns
> the IRQ on in write_unlock_irq(), causes this warning.
>
> Switch it ot write_lock_irqsave() and write_unlock_irqrestore(),
> as suggested by Eric.
>
> Reported-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Signed-off-by: WANG Cong <xiyou.wangcong@...il.com>
>
> ----
>
> diff --git a/fs/fcntl.c b/fs/fcntl.c
> index 97e01dc..556b404 100644
> --- a/fs/fcntl.c
> +++ b/fs/fcntl.c
> @@ -199,7 +199,8 @@ static int setfl(int fd, struct file * filp, unsigned long arg)
>  static void f_modown(struct file *filp, struct pid *pid, enum pid_type type,
>                       int force)
>  {
> -	write_lock_irq(&filp->f_owner.lock);
> +	int flags;

Minor nit.  This should be "unsigned long flags;"

> +	write_lock_irqsave(&filp->f_owner.lock, flags);
>  	if (force || !filp->f_owner.pid) {
>  		put_pid(filp->f_owner.pid);
>  		filp->f_owner.pid = get_pid(pid);
> @@ -211,7 +212,7 @@ static void f_modown(struct file *filp, struct pid *pid, enum pid_type type,
>  			filp->f_owner.euid = cred->euid;
>  		}
>  	}
> -	write_unlock_irq(&filp->f_owner.lock);
> +	write_unlock_irqrestore(&filp->f_owner.lock, flags);
>  }
>  
>  int __f_setown(struct file *filp, struct pid *pid, enum pid_type type,

Eric