linux-kernel - Re: patch "TTY: remove tty

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Date:	Wed, 24 Aug 2011 13:20:47 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	gregkh@...e.de, alan@...ux.intel.com,
	Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: patch "TTY: remove tty_locked" added to tty tree

On Wednesday 24 August 2011, Jiri Slaby wrote:
> On 08/24/2011 10:46 AM, Arnd Bergmann wrote:
> > On Tuesday 23 August 2011 20:54:08 Jiri Slaby wrote:
> >> On 08/23/2011 08:46 PM, Arnd Bergmann wrote:

> > I think I just saw another problem: uart_close takes port->mutex while
> > holding the BTM, then calls tty_wait_until_sent(). If this releases
> > and reaquires the BTM, you get an AB-BA deadlock with port->mutex.
> 
> Aargh, right. The question is why uart_close takes port->mutex there? It
> may take it even right before uart_shutdown. As tty_wait_until_sent (or
> uart_wait_until_sent) may be called e.g. from set_termios without that
> lock anyway. There are ->tx_empty and ->stop_rx that may need some
> protection. But those are register accessors, so they should be
> protected by some spinlock to not race with interrupts. Actually stop_rx
> is. And empty_rx is only in 8250.
> 
> And I don't see anything else there which would need be protected by the
> lock. Do you?

I have not looked at correctness of port->lock before, I just tried to
make sure that BTM correctly nests around it when I removed the BKL.

It's not clear to me what state->mutex protects in the serial_core, but
it has been around forever (used to be called state->sem) and is held in
all uart functions, which is at least consistent. IIRC what Alan's plan
for this was, uart_close should eventually get changed to use
tty_port_close_start or even tty_port_close. Maybe the time for that has
come now, lacking better alternatives?

A lot of other drivers call tty_port_close_start() before taking port->mutex.

	Arnd
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