linux-kernel - Re: [PATCH 3/4] tty: Lock tty pair in tty_init

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Message-ID: <1535718146.23560.80.camel@arista.com>
Date:   Fri, 31 Aug 2018 13:22:26 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     Jiri Slaby <jslaby@...e.cz>, linux-kernel@...r.kernel.org
Cc:     Daniel Axtens <dja@...ens.net>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Tan Xiaojun <tanxiaojun@...wei.com>,
        Peter Hurley <peter@...leysoftware.com>,
        Pasi Kärkkäinen <pasik@....fi>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Michael Neuling <mikey@...ling.org>,
        Mikulas Patocka <mpatocka@...hat.com>
Subject: Re: [PATCH 3/4] tty: Lock tty pair in tty_init_dev()

On Fri, 2018-08-31 at 08:54 +0200, Jiri Slaby wrote:
> On 08/29/2018, 06:28 PM, Dmitry Safonov wrote:
> > On Wed, 2018-08-29 at 16:46 +0200, Jiri Slaby wrote:
> > > On 08/29/2018, 04:23 AM, Dmitry Safonov wrote:
> > > > It's safe to not lock both here - done to silence attempt
> > > > lockdep
> > > > assert in
> > > > tty_ldisc_open(), which will be added with following patch.
> > > 
> > > SOrry, could you elaborate here? I don't follow...
> > 
> > Sure, 4/4 patch adds lockdep_assert_held() into tty_ldisc_open().
> > Currently ldisc in tty->link isn't locked, which according to code
> > shouldn't be an issue, as far as I can see.
> > 
> > So, this patch silences lockdep warining by holding the semaphore,
> > which is slowpath anyway and doesn't case any new contention.
> 
> Eh, no... Adding a lock just to make lockdep happy is a no-go. The
> locking in tty is already complex enough.
> 
> > (actually, not holding the semaphore for slave might be an issue if
> > one
> > opens slave before it's fully initialized, but I'm not sure if it's
> > possible).
> 
> If that turns out to be a problem, we can apply the patch. BUt unless
> it
> is proven to be so (be it code review or a reproducer), let's leave
> the
> locking as it is.

Ok, than for v2 I'll just remove lockdep_assert() from
tty_ldisc_open().

-- 
Thanks,
             Dmitry