linux-kernel - Re: [PATCH v3 12/23] tty: Kick waiters _after

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Date:	Thu, 07 Feb 2013 22:16:58 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alan Cox <alan@...ux.intel.com>,
	Sasha Levin <levinsasha928@...il.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ilya Zykov <ilya@...x.ru>, Dave Jones <davej@...hat.com>
Subject: Re: [PATCH v3 12/23] tty: Kick waiters _after_ the ldisc is locked

On Thu, 2013-02-07 at 16:47 +0100, Jiri Slaby wrote:
> On 02/05/2013 09:20 PM, Peter Hurley wrote:
> 
> The question is obvious: why?
> 
> The waiters usually don't care about ldisc.

This patch really needed a decent commit message, but this isn't a fix,
per se.

While I was auditing the ldisc hangup, I realized that when a reader is
woken from sleeping in n_tty_read() and loops back around because there
is more room in the reader's buffer (nr > 0), input_available_p() will
schedule buffer work and wait for it to complete before testing if the
tty is hung up.

This change ensures that flush_to_ldisc() sees a halted ldisc and
promptly returns.  A blocking reader whose read_buf is empty is the most
likely state when a hangup occurs.

I didn't include it tty_ldisc_halt(), because tty_set_ldisc() can't kick
readers out of their i/o loops and tty_ldisc_release() has no waiters.

Even if the change is nixed, the comments should be changed; the FIXME
is from before waiting for readers to exit their i/o loops.

---- separate but related discussion ----

Another thing to note is this really does nothing to address the problem
of a slave pty being unable to read all the data written by the master,
if the master is closed with a lot of data in the tty buffer. In this
case, when the slave is hungup as a result of the master closing, it's
ldisc will be halted, preventing the tty_buffer from pushing more data. 

In my opinion, this is an important fixme.  Almost as important as
replacing the TTY_LDISC flag + reference count with a rwsem to get
lockdep support...

> > Signed-off-by: Peter Hurley <peter@...leysoftware.com>
> > ---
> >  drivers/tty/tty_ldisc.c | 12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
> > index c903778..8a86a72 100644
> > --- a/drivers/tty/tty_ldisc.c
> > +++ b/drivers/tty/tty_ldisc.c
> > @@ -604,6 +604,12 @@ static bool tty_ldisc_hangup_halt(struct tty_struct *tty)
> >  
> >  	clear_bit(TTY_LDISC, &tty->flags);
> >  
> > +	/* Wakeup waiters so they can discover the tty is hupping, abort
> > +	 * their i/o loops, and release their ldisc references
> > +	 */
> > +	wake_up_interruptible_poll(&tty->write_wait, POLLOUT);
> > +	wake_up_interruptible_poll(&tty->read_wait, POLLIN);
> > +
> >  	if (tty->ldisc) {	/* Not yet closed */
> >  		tty_unlock(tty);
> >  
> > @@ -875,12 +881,6 @@ void tty_ldisc_hangup(struct tty_struct *tty)
> >  		tty_ldisc_deref(ld);
> >  	}
> >  	/*
> > -	 * FIXME: Once we trust the LDISC code better we can wait here for
> > -	 * ldisc completion and fix the driver call race
> > -	 */
> > -	wake_up_interruptible_poll(&tty->write_wait, POLLOUT);
> > -	wake_up_interruptible_poll(&tty->read_wait, POLLIN);
> > -	/*
> >  	 * Shutdown the current line discipline, and reset it to
> >  	 * N_TTY if need be.
> >  	 *
> > 
> 
> 

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