linux-kernel - Re: [PATCH] tty: Only hangup once

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Date:	Sun, 17 Nov 2013 20:38:50 +0300
From:	Heorhi Valakhanovich <valahanovich@....by>
To:	linux-kernel@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: Only hangup once

On Wed, 31 Jul 2013 14:05:45 -0400
Peter Hurley <peter@...leysoftware.com> wrote:

> Instrumented testing shows a tty can be hungup multiple times [1].
> Although concurrent hangups are properly serialized, multiple
> hangups for the same tty should be prevented.
> 
> If tty has already been HUPPED, abort hangup. Note it is not
> necessary to cleanup file *redirect on subsequent hangups,
> as only TIOCCONS can set that value and ioctls are disabled
> after hangup.
> 
> Signed-off-by: Peter Hurley <peter@...leysoftware.com>
> 
> [1]
> Test performed by simulating a concurrent async hangup via
> tty_hangup() with a sync hangup via tty_vhangup(), while
> __tty_hangup() was instrumented with:
> 
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index 26bb78c..fe8b061 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -629,6 +629,8 @@ static void __tty_hangup(struct tty_struct *tty,
> int exit_session)
> 
>  	tty_lock(tty);
> 
> +	WARN_ON(test_bit(TTY_HUPPED, &tty->flags));
> +
>  	/* some functions below drop BTM, so we need this bit */
>  	set_bit(TTY_HUPPING, &tty->flags);
> 
> Test result:
> 
> WARNING: at /home/peter/src/kernels/mainline/drivers/tty/tty_io.c:632
> __tty_hangup+0x459/0x460() Modules linked in: ip6table_filter
> ip6_tables ebtable_nat <...snip...> CPU: 6 PID: 1197 Comm:
> kworker/6:2 Not tainted 3.10.0-0+rfcomm-xeon #0+rfcomm Hardware name:
> Dell Inc. Precision WorkStation T5400  /0RW203, BIOS A11 04/30/2012
> Workqueue: events do_tty_hangup 0000000000000009 ffff8802b16d7d18
> ffffffff816b553e ffff8802b16d7d58 ffffffff810407e0 ffff880254f95c00
> ffff880254f95c00 ffff8802bfd92b00 ffff8802bfd96b00 ffff880254f95e40
> 0000000000000180 ffff8802b16d7d68 Call Trace:
>  [<ffffffff816b553e>] dump_stack+0x19/0x1b
>  [<ffffffff810407e0>] warn_slowpath_common+0x70/0xa0
>  [<ffffffff8104082a>] warn_slowpath_null+0x1a/0x20
>  [<ffffffff813fb279>] __tty_hangup+0x459/0x460
>  [<ffffffff8107409c>] ? finish_task_switch+0xbc/0xe0
>  [<ffffffff813fb297>] do_tty_hangup+0x17/0x20
>  [<ffffffff8105fd6f>] process_one_work+0x16f/0x450
>  [<ffffffff8106007c>] process_scheduled_works+0x2c/0x40
>  [<ffffffff8106060a>] worker_thread+0x26a/0x380
>  [<ffffffff810603a0>] ? rescuer_thread+0x310/0x310
>  [<ffffffff810698a0>] kthread+0xc0/0xd0
>  [<ffffffff816b0000>] ? destroy_compound_page+0x65/0x92
>  [<ffffffff810697e0>] ? kthread_create_on_node+0x130/0x130
>  [<ffffffff816c495c>] ret_from_fork+0x7c/0xb0
>  [<ffffffff810697e0>] ? kthread_create_on_node+0x130/0x130
> ---[ end trace 98d9f01536cf411e ]---
> ---
>  drivers/tty/tty_io.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index 26bb78c..a9355ce 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -629,6 +629,11 @@ static void __tty_hangup(struct tty_struct *tty,
> int exit_session) 
>  	tty_lock(tty);
>  
> +	if (test_bit(TTY_HUPPED, &tty->flags)) {
> +		tty_unlock(tty);
> +		return;
> +	}
> +
>  	/* some functions below drop BTM, so we need this bit */
>  	set_bit(TTY_HUPPING, &tty->flags);
>  

After upgrading to kernel 3.12 I noticed one issue with tmux software.
The easiest way to reproduce will be:
1. Start tmux session as root.
2. Connect via ssh and use "tmux attach" to attach to the running
session.
3. Kill ssh client.

You can notice that shell (zsh in my case) and "tmux attach" are still
remains in process' list. That didn't happen in previous kernels.
I've tried to bisect this in kernel sources and found commit
cb50e5235b8ae5aa0fe422eaaa8e444024c5bd98 which contains this exact
patch. I have not enough experience to investigate more so most likely
I will not find anything more. But it will be good if someone more
experienced will have a look at it.

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