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

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Date:	Fri, 2 Aug 2013 11:46:47 +0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Jiri Slaby <jslaby@...e.cz>, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH] tty: Only hangup once

On Wed, Jul 31, 2013 at 02:05:45PM -0400, Peter Hurley 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);

A diff inside the changelog entry of a diff is going to cause havoc :)

I'll go edit this to make it not complain...

thanks,

greg k-h
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