linux-kernel - Re: [PATCH v3 00/10] BKL conversion in tty layer

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Message-ID: <20100617220926.GC5345@nowhere>
Date:	Fri, 18 Jun 2010 00:09:28 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Tony Luck <tony.luck@...el.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org, Greg KH <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	John Kacur <jkacur@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH v3 00/10] BKL conversion in tty layer

On Thu, Jun 17, 2010 at 11:48:07PM +0200, Arnd Bergmann wrote:
> On Thursday 17 June 2010 22:15:32 Tony Luck wrote:
> > Call Trace:
> >  [<a000000100015990>] show_stack+0x50/0xa0
> >  [<a00000010090f1f0>] dump_stack+0x30/0x50
> >  [<a00000010008e280>] warn_slowpath_common+0xc0/0x120
> >  [<a00000010008e320>] warn_slowpath_null+0x40/0x60
> >  [<a00000010053d910>] tty_release+0x90/0xbc0
> >  [<a0000001001ab200>] __fput+0x260/0x420
> >  [<a0000001001ab400>] fput+0x40/0x60
> >  [<a00000010053b3b0>] tty_vhangup_locked+0x870/0x8a0
> >  [<a00000010054f3f0>] pty_close+0x350/0x3a0
> >  [<a00000010053ddd0>] tty_release+0x550/0xbc0
> >  [<a0000001001ab200>] __fput+0x260/0x420
> >  [<a0000001001ab400>] fput+0x40/0x60
> >  [<a0000001001a4dc0>] filp_close+0x120/0x140
> >  [<a0000001001a4f90>] sys_close+0x1b0/0x2c0
> >  [<a00000010000b940>] ia64_ret_from_syscall+0x0/0x20
> > 
> 
> Ah, this sucks. I think Alan actually tried to warn me of this problem and I
> thought I had it right, but obviously I got it wrong in the end. I really
> should have run into this during testing though, not sure why I didn't.
> 
> The good news is that the warning message is harmless for the normal
> case where CONFIG_TTY_MUTEX remains disabled, it's only debugging code
> to warn that there is a bug once the option gets turned on.
> 
> Unfortunately however the only fix I see is to push the BTM further down
> into the hangup function, which makes the pty code slightly more complex
> and which I'm sure is an equivalent transformation.
> 
> I'll try doing some more tests with this patch and CONFIG_TTY_LOCK disabled.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> 
> diff --git a/drivers/char/pty.c b/drivers/char/pty.c
> index c9af9ff..0902127 100644
> --- a/drivers/char/pty.c
> +++ b/drivers/char/pty.c
> @@ -62,7 +62,9 @@ static void pty_close(struct tty_struct *tty, struct file *filp)
>  		if (tty->driver == ptm_driver)
>  			devpts_pty_kill(tty->link);
>  #endif
> -		tty_vhangup_locked(tty->link);
> +		unlock_kernel();
> +		tty_vhangup(tty->link);
> +		lock_kernel();



Don't you mean tty_unlock / tty_lock there?

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