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

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Date:	Wed, 24 Nov 2010 22:36:00 +0000
From:	Alan Cox <alan@...ux.intel.com>
To:	Alexander Gordeev <lasaine@....cs.msu.su>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, 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>,
	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

> Yes, indeed, it's a bug. Please consider the following example:
> 
>      CPU1                                      CPU2
> =========================================================
> uart_handle_dcd_change() {		tty_set_ldisc() {
>   ld = tty_ldisc_ref(...)			  ...

[We have a reference]
>   ...					  tty_ldisc_halt(...)
					  [Should block]

> I think tty code is exactly the right place to fix this bug; this is
> what my patch is for.

More special case magic on top of the current crap isn't the right fix
here, tty_ldisc_halt needs to wait for the references to hit zero.

Alan
 

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