linux-kernel - Re: [PATCHv5 05/17] tty: don't allow ldisc dcd

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Message-ID: <20101124164329.75368d71@lxorguk.ukuu.org.uk>
Date:	Wed, 24 Nov 2010 16:43:29 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Alexander Gordeev <lasaine@....cs.msu.su>
Cc:	linux-kernel@...r.kernel.org,
	"Nikita V\. Youshchenko" <yoush@...msu.su>,
	linuxpps@...enneenne.com, Rodolfo Giometti <giometti@...eenne.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Arnd Bergmann <arnd@...db.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Nick Piggin <npiggin@...nel.dk>,
	"Alan \"I must be out of my tree\" Cox" <alan@...ux.intel.com>,
	Jason Wessel <jason.wessel@...driver.com>,
	Philippe Langlais <philippe.langlais@...ricsson.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCHv5 05/17] tty: don't allow ldisc dcd_change() after ldisc
 halt

On Wed, 24 Nov 2010 19:15:43 +0300
Alexander Gordeev <lasaine@....cs.msu.su> wrote:

> There was a possibility that uart_handle_dcd_change() could obtain a
> reference to ldisc while running in parallel with tty_set_ldisc() on
> different CPU but call dcd_change() operation after tty_ldisc_close()
> which is incorrect.

How can this occur ?


> +	spin_lock_irqsave(&tty->dcd_change_lock, flags);
> +
> +	ld = tty_ldisc_ref(tty);

What is the expecting lock ordering rule here ?



I don't see why this patch is needed. You've got an ldisc ref from
tty_ldisc_ref, until you drop that ldisc ref you are fine. If for some
reason that is not the case then there is a bug in the ldisc code.

Alan
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