linux-kernel - Re: pty tcdrain() bug in 2.6.27 to 2.6.30/current

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Message-Id: <200906100153.38925.luke@dashjr.org>
Date:	Wed, 10 Jun 2009 01:53:20 -0500
From:	"Luke-Jr" <luke@...hjr.org>
To:	Mike Galbraith <gleep@....de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: pty tcdrain() bug in 2.6.27 to 2.6.30/current

On Wednesday 10 June 2009 01:39:14 am Mike Galbraith wrote:
> On Tue, 2009-06-09 at 23:38 -0500, Luke-Jr wrote:
> > tcdrain blocks on ptys since 2.6.27; at least 2.6.26 returns in a timely
> > manner. The following test case reproduces the bug, and hangs only on
> > affected kernels. Examination of 2.6.26 and 2.6.27 suggests the ioctl
> > used by tcdrain underwent a rewrite for 2.6.27, and thus fixing this bug
> > is beyond my capabilities at this time.
>
> It looks to me like it's doing what it should do, and any bug would be
> in kernels before the break handling (by Alan Cox, CC'd) rework.
>
> Disclaimer:  My knowledge wrt tty IO approaches 0, but since I burned a
> bit of time rummaging, you get one absolutely free reply.

My knowledge on how it *should* work also approaches 0, but I am unfortunately 
faced with a proprietary (user-space) blob GPS driver (which never reads its 
NMEA-providing pty) that relies on this behaviour (or gpsd not doing the 
tcdrain). However, the manual page for tcdrain says it waits until data is 
*trasmitted*, not necessarily received... Not sure if that makes a difference 
for this scenario?

Luke
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