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

Jiri Slaby
2011-08-24T09:31:32Z

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:
>>> According to http://kernelnewbies.org/BigKernelLock, I concluded back then
>>> that tty_wait_until_sent would always be called without BTM held. Has that
>>> changed recently, or did I miss a caller that holds the BTM?
>>
>> Every tty_operations->close and ->hangup :).
>
> Ah, right, I remember. The chart I did was only to prove that locking was
> consistent (i.e. no deadlocks), it ignored that the function needs to be
> called without BTM because I had incorrectly convinced myself that the
> wait_event_interruptible_timeout() didn't need to release it.
>
> 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?

thanks,
--
js
suse labs