linux-kernel - Re: tty crash in tty_ldisc_receive

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Message-ID: <1491462988.4166.104.camel@kernel.crashing.org>
Date:   Thu, 06 Apr 2017 17:16:28 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Michael Neuling <mikey@...ling.org>,
        Al Viro <viro@...IV.linux.org.uk>,
        johan Hovold <johan@...nel.org>,
        Peter Hurley <peter@...leysoftware.com>,
        Wang YanQing <udknight@...il.com>,
        Alexander Popov <alex.popov@...ux.com>,
        Rob Herring <robh@...nel.org>
Cc:     Mikulas Patocka <mpatocka@...hat.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: tty crash in tty_ldisc_receive_buf()

On Thu, 2017-04-06 at 17:04 +1000, Michael Neuling wrote:
> We see this usually on boot but can also see it if we kill a getty attached to
> tty (which is then respawned by systemd).  It seems like we are flushing data to
> a tty at the same time as it's being torn down and restarted.
> 
> I did try the below patch which avoids the crash but locks up one of the CPUs. I
> guess the data never gets flushed if we say nothing is processed.
> 
> This is on powerpc but has also been reported by parisc.
> 
> I'm not at all familiar with the tty layer and looking at the locks, mutexes,
> semaphores and reference counting in there scares the hell out of me. 
> 
> If anyone has an idea, I'm happy to try a patch.

Note that we noticed one path that called reinit without the ldisc lock
held for writing, we added that, but it didn't fix the problem.

Cheers,
Ben.