linux-kernel - Re: [PATCHv5 0/7] tty: Hold write ldisc sem in tty

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Message-ID: <20181015133751.GO5318@reaktio.net>
Date:   Mon, 15 Oct 2018 16:37:51 +0300
From:   Pasi Kärkkäinen <pasik@....fi>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Daniel Axtens <dja@...ens.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Michael Neuling <mikey@...ling.org>,
        Nathan March <nathan@...net>,
        Peter Hurley <peter@...leysoftware.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rong, Chen" <rong.a.chen@...el.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tan Xiaojun <tanxiaojun@...wei.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Jiri Slaby <jslaby@...e.cz>,
        syzbot+3aa9784721dfb90e984d@...kaller.appspotmail.com
Subject: Re: [PATCHv5 0/7] tty: Hold write ldisc sem in tty_reopen()

Hi,

On Thu, Sep 20, 2018 at 10:25:01AM -0400, Mikulas Patocka wrote:
> 
> 
> On Wed, 19 Sep 2018, Dmitry Safonov wrote:
> 
> > On Wed, 2018-09-19 at 16:03 -0400, Mikulas Patocka wrote:
> > > 
> > > On Wed, 19 Sep 2018, Dmitry Safonov wrote:
> > > > 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.
> > 
> > Thanks much for that again.
> > Any chance you have sysrq enabled and could print: locks held in system
> > and kernel backtraces for applications?
> 
> Stacktrace doesn't work on parisc. It used to work on 4.18, but the kernel
> 4.19 reportedly requires patched binutils.
> 
> Mikulas
> 
> > I suppose, userspace hangs in n_tty_receive_buf_common(), which is
> > better of cause than null-ptr dereference, but I believe we can improve
> > it by stopping a reader earlier if there is any writer pending.
> > 
> > -- 
> > Thanks,
> >              Dmitry
> > 

Any progress here? Or is more info/stacktrace needed about the hang?


Thanks,

-- Pasi