Re: [PATCH RESEND] tty: don't dead lock while flushing workqueue

[Peter Hurley <peter@hurleysoftware.com>]

On Tue, 2012-11-27 at 19:01 +0100, Sebastian Andrzej Siewior wrote:
> Since commit 89c8d91e31f2 ("tty: localise the lock") I see a dead lock
> in one of my dummy_hcd + g_nokia test cases. The first run one was usually
> okay, the second often resulted in a splat by lockdep and the third was
> usually a dead lock.
....
> 
> Before the path mentioned tty_ldisc_release() look like this:
> 
> |	tty_ldisc_halt(tty);
> |	tty_ldisc_flush_works(tty);
> |	tty_lock();
> 
> As it can be seen, it first flushes the workqueue and then grabs the
> tty_lock. Now we grab the lock first:
> 
> |	tty_lock_pair(tty, o_tty);
> |	tty_ldisc_halt(tty);
> |	tty_ldisc_flush_works(tty);
> 
> so lockdep's complaint seems valid.
> 
> The other user of tty_ldisc_flush_works() is tty_set_ldisc() and I tried
> to mimnic its logic:

The lock logic for tty_set_ldisc() is wrong. Despite existing code in
tty_set_ldisc() and tty_ldisc_hangup(), the ldisc_mutex does **not**
(and should not) play a role in acquiring or releasing ldisc references.
The only thing that needs to happen here is below (don't actually use
below because I just hand-edited it):

> See http://lkml.org/lkml/2012/11/21/347
> 
>  drivers/tty/tty_ldisc.c |   13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
> index 0f2a2c5..fb76818 100644
> --- a/drivers/tty/tty_ldisc.c
> +++ b/drivers/tty/tty_ldisc.c
> @@ -930,16 +930,21 @@ void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)
>  	 */
>  
> - 	tty_lock_pair(tty, o_tty);
>  	tty_ldisc_halt(tty);
> 	tty_ldisc_flush_works(tty);

 
> +	tty_lock_pair(tty, o_tty);
>  	/* This will need doing differently if we need to lock */
>  	tty_ldisc_kill(tty);
> -
>  	if (o_tty)
>  		tty_ldisc_kill(o_tty);
>