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Date:   Wed, 19 Sep 2018 16:03:41 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     Dmitry Safonov <dima@arista.com>
cc:     linux-kernel@vger.kernel.org,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        Daniel Axtens <dja@axtens.net>,
        Dmitry Vyukov <dvyukov@google.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Michael Neuling <mikey@neuling.org>,
        Nathan March <nathan@gt.net>,
        =?ISO-8859-15?Q?Pasi_K=E4rkk=E4inen?= <pasik@iki.fi>,
        Peter Hurley <peter@hurleysoftware.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Rong, Chen" <rong.a.chen@intel.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Tan Xiaojun <tanxiaojun@huawei.com>,
        Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        stable@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jslaby@suse.com>, Jiri Slaby <jslaby@suse.cz>,
        syzbot+3aa9784721dfb90e984d@syzkaller.appspotmail.com
Subject: Re: [PATCHv5 0/7] tty: Hold write ldisc sem in tty_reopen()
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On Wed, 19 Sep 2018, Dmitry Safonov wrote:

> On Wed, 2018-09-19 at 13:35 -0400, Mikulas Patocka wrote:
> > 
> > On Wed, 19 Sep 2018, Mikulas Patocka wrote:
> > 
> > > 
> > > 
> > > On Tue, 18 Sep 2018, Dmitry Safonov wrote:
> > > 
> > > > Hi all,
> > > > 
> > > > Three fixes that worth to have in the @stable, as they were hit
> > by
> > > > different people, including Arista on v4.9 stable.
> > > > 
> > > > And for linux-next - adding lockdep asserts for line discipline
> > changing
> > > > code, verifying that write ldisc sem will be held forthwith.
> > > > 
> > > > The last patch is an optional and probably, timeout can be
> > dropped for
> > > > read_lock(). I'll do it if everyone agrees.
> > > > (Or as per discussion with Peter in v3, just convert ldisc to
> > > > a regular rwsem).
> > > > 
> > > > Thanks,
> > > > Dima
> > > 
> > > I confirm that this patch series fixes the crash for me.
> > > 
> > > Tested-by: Mikulas Patocka <mpatocka@redhat.com>
> > 
> > I was too quick to acknowledge this patchset. It doesn't work.
> > 
> > This patchset fixes the crash, but it introduces another bug - when I
> > type 
> > 'reboot' on the console, it prints 'INIT: Switching to runlevel: 6'
> > and 
> > then it gets stuck for 80 seconds before proceeding with the reboot.
> > When 
> > I revert this patchset 'reboot' reboots the machine without any
> > delay. 
> > This bug was reproduced on Debian 5 userspace on pa-risc.
> 
> Thanks much for the testing, Mikulas.
> Could you try to bisect which of the patches causes it?
> The most important are 1,2,3 - probably, one of them caused it..
> I'll stare a bit into the code.

The patch 3 causes it.

The hangs during reboot take either 80 seconds, 3 minutes or 3:25.

Mikulas