Re: patch "TTY: remove tty_locked" added to tty tree

[Arnd Bergmann <arnd@arndb.de>]

On Wednesday 24 August 2011, Jiri Slaby wrote:
> On 08/24/2011 01:20 PM, Arnd Bergmann wrote:
> > 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)
> 
> It was actually moved in uart_close back in 2003. Formerly (when there
> was only a coarse grained port_sem) it was right before uart_shutdown.
> But there were some flags to handle some races. I'm not sure whether the
> flags protected any race here though.

ok

> > 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?
> 
> Yes, I have such a patch in my queue. But it's not easy to get there.
> You may take a look at:
> http://decibel.fi.muni.cz/gitweb/?p=linux.git;a=shortlog;h=refs/heads/devel
> 
> But it's still far from ready. And yet, in the queue, I still have
> port->mutex locked before tty_port_close_start like it is now.

Ah, right. I still don't see why the port->mutex is or is not needed there,
and I think that's the main issue.

By comparison, getting *_wait_until_sent to be called without BTM seems
easy -- we know that all callers from ->close() hold it, while the ones
from ->ioctl() don't. We could have a helper like

void tty_wait_until_sent_from_close(struct tty_struct *tty, long timeout)
{
	tty_unlock(); /* tty->ops->close holds the BTM, drop it while waiting */
	tty_wait_until_sent(tty, timeout);
	tty_lock();
}

to deal with that, if only we can sort the lock ordering with port->mutex.

BTW, I saw that the three m68k serial port drivers (amiserial, 68328, 68360)
all call *_wait_until_sent with interrupts disabled, which is even more
broken.

	Arnd