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Date:   Wed, 19 Sep 2018 12:56:37 -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>,
        Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        syzbot+3aa9784721dfb90e984d@syzkaller.appspotmail.com
Subject: Re: [PATCHv5 0/7] tty: Hold write ldisc sem in tty_reopen()
In-Reply-To: <20180917235258.5719-1-dima@arista.com>
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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>


> Changes since v4:
> - back to lock ldisc with (5*HZ) timeout in tty_reopen()
>   (LKP report link: lkml.kernel.org/r/<1536940609.3185.29.camel@arista.com>)
> - reordered 3/7 with 2/7 for LKP robot
> 
> Changes since v3:
> - Added tested-by Mark Rutland (thanks!)
> - Dropped patch with smp_wmb() - wrong idea
> - lockdep_assert_held() should be actually lockdep_assert_held_exclusive()
> - Described why tty_ldisc_open() can be called without ldisc_sem held
>   for pty slave end (o_tty).
> - Added Peter's patch for dropping self-made lockdep annotations
> - Fix for a reader(s) of ldisc semaphore waiting for an active reader(s)
> 
> Changes since v2:
> - Added reviewed-by tags
> - Hopefully, fixed reported by 0-day issue.
> - Added optional fix for wait_readers decrement
> 
> Changes since v1:
> - Added tested-by/reported-by tags
> - Dropped 3/4 (locking tty pair for lockdep sake),
>   Because of that - not adding lockdep_assert_held() in tty_ldisc_open()
> - Added 4/4 cleanup to inc tty->count only on success of
>   tty_ldisc_reinit()
> - lock ldisc without (5*HZ) timeout in tty_reopen()
> 
> v1 link: lkml.kernel.org/r/<20180829022353.23568-1-dima@arista.com>
> v2 link: lkml.kernel.org/r/<20180903165257.29227-1-dima@arista.com>
> v3 link: lkml.kernel.org/r/<20180911014821.26286-1-dima@arista.com>
> v4 link: lkml.kernel.org/r/<20180912001702.18522-1-dima@arista.com>
> 
> Cc: Daniel Axtens <dja@axtens.net>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Michael Neuling <mikey@neuling.org>
> Cc: Mikulas Patocka <mpatocka@redhat.com>
> Cc: Nathan March <nathan@gt.net>
> Cc: Pasi Kärkkäinen <pasik@iki.fi>
> Cc: Peter Hurley <peter@hurleysoftware.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: "Rong, Chen" <rong.a.chen@intel.com>
> Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
> Cc: Tan Xiaojun <tanxiaojun@huawei.com>
> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> (please, ignore if I Cc'ed you mistakenly)
> 
> Dmitry Safonov (6):
>   tty: Drop tty->count on tty_reopen() failure
>   tty/ldsem: Wake up readers after timed out down_write()
>   tty: Hold tty_ldisc_lock() during tty_reopen()
>   tty: Simplify tty->count math in tty_reopen()
>   tty/ldsem: Add lockdep asserts for ldisc_sem
>   tty/ldsem: Decrement wait_readers on timeouted down_read()
> 
> Peter Zijlstra (1):
>   tty/ldsem: Convert to regular lockdep annotations
> 
>  drivers/tty/tty_io.c    | 13 ++++++++---
>  drivers/tty/tty_ldisc.c |  9 +++++++
>  drivers/tty/tty_ldsem.c | 62 ++++++++++++++++++++-----------------------------
>  3 files changed, 44 insertions(+), 40 deletions(-)
> 
> -- 
> 2.13.6
> 
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