linux-kernel - Re: [PATCH v2 20/20] tty: Remove extra wakeup from pty write() path

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Date:	Sat, 20 Jul 2013 13:00:47 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	Peter Hurley <peter@...leysoftware.com>,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	Jiri Slaby <jslaby@...e.cz>,
	Andre Naujoks <nautsch2@...il.com>,
	Dean Jenkins <Dean_Jenkins@...tor.com>
Subject: Re: [PATCH v2 20/20] tty: Remove extra wakeup from pty write() path

On 06/15/2013 10:21 AM, Peter Hurley wrote:
> Acquiring the write_wait queue spin lock now accounts for the largest
> slice of cpu time on the tty write path. Two factors contribute to
> this situation; a overly-pessimistic line discipline write loop which
> _always_ sets up a wait loop even if i/o will immediately succeed, and
> on ptys, a wakeup storm from reads and writes.
>
> Writer wakeup does not need to be performed by the pty driver.
> Firstly, since the actual i/o is performed within the write, the
> line discipline write loop will continue while space remains in
> the flip buffers. Secondly, when space becomes avail in the
> line discipline receive buffer (and thus also in the flip buffers),
> the pty unthrottle re-wakes the writer (non-flow-controlled line
> disciplines unconditionally unthrottle the driver when data is
> received). Thus, existing in-kernel i/o is guaranteed to advance.
> Finally, writer wakeup occurs at the conclusion of the line discipline
> write (in tty_write_unlock()). This guarantees that any user-space write
> waiters are woken to continue additional i/o.

Greg,

I thought I should let you know I'm tracking down a bug/regression
related to this patch.

In certain unusual pty/ldisc configurations, i/o fails to make
forward progress. I still stand by my commit message above, so I'm
in the process of instrumenting the i/o path so I can uncover the
cause of the failure.

I would still recommend applying this patch to tty-next, as it
resolves a much more critical bug discussed here [1].

Doing a write_wakeup() from a driver .write() method is a no-no;
recursion is possible and results in a thread stack overrun.

Regards,
Peter Hurley

[1]
https://lkml.org/lkml/2013/7/1/308

> Signed-off-by: Peter Hurley <peter@...leysoftware.com>
> ---
>   drivers/tty/pty.c | 4 +---
>   1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
> index 0634dd9..b9bc5be 100644
> --- a/drivers/tty/pty.c
> +++ b/drivers/tty/pty.c
> @@ -121,10 +121,8 @@ static int pty_write(struct tty_struct *tty, const unsigned char *buf, int c)
>   		/* Stuff the data into the input queue of the other end */
>   		c = tty_insert_flip_string(to->port, buf, c);
>   		/* And shovel */
> -		if (c) {
> +		if (c)
>   			tty_flip_buffer_push(to->port);
> -			tty_wakeup(tty);
> -		}
>   	}
>   	return c;
>   }
>

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