linux-kernel - Re: commit 83d817f410 broke my ability to use Linux with a braille display

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Message-ID: <20190111191155.s4e5sn5fqk33fqr4@shells.gnugeneration.com>
Date:   Fri, 11 Jan 2019 11:11:55 -0800
From:   Vito Caputo <vcaputo@...garu.com>
To:     Nicolas Pitre <nico@...xnic.net>
Cc:     Dmitry Safonov <dima@...sta.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Mark Rutland <mark.rutland@....com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Tycho Andersen <tycho@...ho.ws>, Dave Mielke <dave@...lke.cc>,
        linux-kernel@...r.kernel.org
Subject: Re: commit 83d817f410 broke my ability to use Linux with a braille
 display

On Fri, Jan 11, 2019 at 01:33:09PM -0500, Nicolas Pitre wrote:
> I use Linux with the help of a braille display and the brltty daemon. It 
> turns out that the latest mainline kernel I can work with comes from 
> commit 231f8fd0cc. Anything past that and I lose the ability to read the 
> console barely a few seconds after the system has booted as brltty is 
> thrown a wrench and the braille display becomes completely inoperable.
> 
> Things get somewhat better with commit c96cf923a9 as brltty is not 
> longer incapacitated, but some programs would randomly crash. Even the 
> very first login attempt won't work as I soon as I hit enter after my 
> user name the password prompt is skipped over, just like if the enter 
> key had been hit twice. Then lynx (the text web browser) would crash as 
> soon as I switch the virtual console with LeftAlt+FN. Mind you, this 
> isn't easy to perform bisection in those conditions.
> 
> And the worst commit i.e. 83d817f410 is marked for stable!  :-(
> 
> Some interaction with brltty must be at play here otherwise such 
> breakage would never have survived up to the mainline kernel.
> 
> As far as latest mainline is concerned, I managed to reproduce at least 
> one of the unwelcome behavior change (hoping that's all there is to this 
> issue) with a very simple test case so you won't have to learn braille 
> to debug this:
> 
> # from any vt, make sure tty40 is allocated and empty
> openvt -c 40 -f -- true
> 
> # open it and wait on read()
> cat /dev/tty40
> 
> # from a second vt, simply open tty40 again
> true < /dev/tty40
> 
> # come back to the first vt and watch cat bailing out with EAGAIN.
> 
> Please fix.
> 
> 
> Nicolas


This all sounds familiar, and I suspect this is the fix:

https://lkml.org/lkml/2019/1/8/1379

Regards,
Vito Caputo