Messages in this thread | | | Date | Fri, 17 May 2013 15:08:26 -0400 | From | Peter Hurley <> | Subject | Re: [PATCH] tty: Add missing lock in n_tty_write() |
| |
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
| |