Re: [RCF/RFT] Fix race timer race in gameport-based joystick drivers

[Vojtech Pavlik <vojtech@suse.cz>]

On Tue, Feb 15, 2005 at 09:51:52AM -0500, Dmitry Torokhov wrote:
> On Tue, 15 Feb 2005 15:05:01 +0100, Vojtech Pavlik <vojtech@suse.cz> wrote:
> > On Tue, Feb 15, 2005 at 12:42:31AM -0500, Dmitry Torokhov wrote:
> > > Hi,
> > >
> > > There seems to be a race WRT to timer handling in all gameport-based
> > > joystick drivers. open() and close() methods are used to start and
> > > stop polling timers on demand but counter and the timer itself is not
> > > protected in any way so if several clients will try to open/close
> > > corresponding input device node they could up with timer not running
> > > at all or running while nobody has the node open. Plus it is possible
> > > that disconnect will run and free driver structure while timer is running
> > > on other CPU.
> > >
> > > I have moved timer and counter down into gameport structure (I think it
> > > is ok because on the one hand joysticks are the only users of gameport
> > > and on the other hand polling timer can be useful to other clients if
> > > ever writen), and added helper functions to manipulate it:
> > >
> > >       - gameport_start_polling(gameport)
> > >       - gameport_stop_polling(gameport)
> > >       - gameport_set_poll_handler(gameoirt, handler)
> > >       - gameport_set_poll_interval(gameport, msecs)
> > >
> > > gameport_{start|stop}_poll handler are using spinlock to guarantee that
> > > timer updated properly. Also, gameport_close deletes (synchronously) timer
> > > to make sure there is no surprises since gameport_stop_poling does del_timer
> > > and thus may leave timer scheduled. Timer routine also checks the counter
> > > and does not restart it if there are no users.
> > >
> > > Please let me know what you think.
> > 
> > I'm not really sure if I really want to move the polling into the
> > gameport layer. It's useful, but without it, gameport is considered
> > strictly a passive device which can't generate callbacks (other than
> > open/close/connect/disconnect).
> > 
> > The new polling interface isn't much simpler than what Linux timers
> > offer, only it provides additional locking.
> 
> Yes, that was the goal. I looked over the drivers and it was either
> writing the exactly same code 10 times or moving it down.

> > Probably protecting open/close calls in gameport.c with a spinlock would
> > allow to work without explicit locking in the drivers.
> 
> Hmm, you got me a bit confused here - open and close in gameport are
> already (indirectly) serialized with gameport_sem. It is input device
> open and close in joystick drivers that needs treatment - these are
> initiated from userspace and weren't hitting gameport code at all. And
> they need to be protected otherwise the counter and timer will get out
> of whack.

Sorry, I was indeed a bit confused - the input open serialization would
be needed, but still the timer could race.

Thinking more about it I agree with your change. 

-- 
Vojtech Pavlik
SuSE Labs, SuSE CR