linux-kernel - Re: [PATCH tty-next 09/22] tty: Remove tty_hung_up

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Date:	Mon, 16 Jun 2014 15:52:23 +0200
From:	Jesper Nilsson <jesper.nilsson@...s.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Mikael Starvik <starvik@...s.com>,
	Jesper Nilsson <jespern@...s.com>,
	Samuel Ortiz <samuel@...tiz.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH tty-next 09/22] tty: Remove tty_hung_up_p() tests from
 tty drivers' open()

On Mon, Jun 16, 2014 at 09:17:06AM -0400, Peter Hurley wrote:
> Since at least before 2.6.30, it has not been possible to observe
> a hung up file pointer in a tty driver's open() method unless/until
> the driver open() releases the tty_lock() (eg., before blocking).
> 
> This is because tty_open() adds the file pointer while holding
> the tty_lock() _and_ doesn't release the lock until after calling
> the tty driver's open() method. [ Before tty_lock(), this was
> lock_kernel(). ]
> 
> Since __tty_hangup() first waits on the tty_lock() before
> enumerating and hanging up the open file pointers, either
> __tty_hangup() will wait for the tty_lock() or tty_open() will
> not yet have added the file pointer. For example,
> 
> CPU 0                          |  CPU 1
>                                |
> tty_open                       |  __tty_hangup
>   ..                           |    ..
>   tty_lock                     |    ..
>   tty_reopen                   |    tty_lock  / blocks
>   ..                           |
>   tty_add_file(tty, filp)      |
>   ..                           |
>   tty->ops->open(tty, filp)    |
>     tty_port_open              |
>       tty_port_block_til_ready |
>         ..                     |
>         while (1)              |
>           ..                   |
>           tty_unlock           |    / unblocks
>           schedule             |    for each filp on tty->tty_files
>                                |      f_ops = tty_hung_up_fops;
>                                |    ..
>                                |    tty_unlock
>           tty_lock             |
>   ..                           |
>   tty_unlock                   |
> 
> Note that since tty_port_block_til_ready() and similar drop
> the tty_lock while blocking, when woken, the file pointer
> must then be tested for having been hung up.
> 
> Also, fix bit-rotted drivers that used extra_count to track the
> port->count bump.
> 
> CC: Mikael Starvik <starvik@...s.com>

For the CRIS part:

Acked-by: Jesper Nilsson <jesper.nilsson@...s.com>

> CC: Samuel Ortiz <samuel@...tiz.org>
> CC: "David S. Miller" <davem@...emloft.net>
> Signed-off-by: Peter Hurley <peter@...leysoftware.com>

/^JN - Jesper Nilsson
-- 
               Jesper Nilsson -- jesper.nilsson@...s.com
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