linux-kernel - Re: tty->low_latency + irq context

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Message-ID: <20070102183829.10d861fc@localhost.localdomain>
Date:	Tue, 2 Jan 2007 18:38:29 +0000
From:	Alan <alan@...rguk.ukuu.org.uk>
To:	Hollis Blanchard <hollisb@...ibm.com>
Cc:	Jiri Slaby <jirislaby@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	p.hardwick@...ion.com
Subject: Re: tty->low_latency + irq context

> with tty->low_latency set, but it doesn't AFAICS. One possibility for
> deadlock is if the tty->buf.lock spinlock is taken on behalf of a user
> process...

The case to watch out for is

	flip_buffer_push -> ldisc -> driver write of echo/^S/^Q

if you call flip_buffer_push while holding your own lock you may get in
a mess on the echo path.
 
>       * data is received, enough to completely fill the tty buffer
>       * tty_flip_buffer_push() schedules flush_to_ldisc()
>       * before flush_to_ldisc() runs, more data is received
>       * flush_to_ldisc() truncates the incoming data (look for
>         tty->receive_room)
> 
> I don't see how this is supposed to work in general.

For non fake tty hardware at real speeds it wasn't a problem under about
1Mbit. Current tty layer code just uses memory buffering based on kmalloc
and has a 64K limit instead. Works better SMP, scales better and we no
longer need to do stunts like the flip buffers to scrape 56Kbit on a
386SX16

Alan
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