Re: [PATCH] tty: Add missing lock in n_tty_write()

[Peter Hurley <peter@hurleysoftware.com>]

On 05/17/2013 07:48 AM, Joerg Roedel wrote:
> Hi Peter,
>
> thanks for you explanations. They helped me to better understand what is
> happening now.
>
> On Wed, May 15, 2013 at 07:10:43PM -0400, Peter Hurley wrote:
>> On 05/15/2013 03:48 PM, Joerg Roedel wrote:
>
>> Agreed. Those functions look written for single-producer/single-consumer
>> i/o model. (That's why I asked about CONFIG_CONSOLE_POLL=y as well because
>> that doesn't look thread-safe either).
>
> Ok, I checked that. CONFIG_CONSOLE_POLL is on in that kernel.
>
>> Just to be clear here: there's a difference between a console driver
>> and a tty driver.
>>
>> The console driver's write() method is serialized with the global
>> console_lock() so parallel console writes are not possible.
>>
>> No such guarantee exists for the tty driver write() method, although it
>> probably wouldn't be difficult to provide that guarantee (since the
>> line discipline write() is already serialized by tty->atomic_write_lock).
>
> Okay, so it is safe to say that currently the drivers write() (and
> put_chars()) functions need to expect to be called concurrently and
> therefore they have to serialize themselves when they need it, right?

If only it were that simple :)

Yes, console write() and tty write() can be concurrent. However, the
console write() can also be _recursive_ wrt. tty write(). This can happen,
for example, if something oopses in the tty write() path.

If you review serial8250_console_write() in drivers/tty/serial/8250/8250_core.c,
you'll see how some of this is worked around.

But looking at this from a wider perspective, the goal should be
to limit the overlap as much as possible.

Regards,
Peter Hurley