linux-kernel - Re: [PATCH] tty: Fix race condition if flushing tty flip buffers

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Date:	Mon, 8 Apr 2013 11:48:04 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Ilya Zykov <linux@...k.ru>, Jiri Slaby <jslaby@...e.cz>,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH] tty: Fix race condition if flushing tty flip buffers

On Wed, Mar 20, 2013 at 01:20:43PM -0400, Peter Hurley wrote:
> As Ilya Zykov identified in his patch 'PROBLEM: Race condition in
> tty buffer's function flush_to_ldisc()', a race condition exists
> which allows a parallel flush_to_ldisc() to flush and free the tty
> flip buffers while those buffers are in-use. For example,
> 
>   CPU 0                         |  CPU 1                                  |  CPU 2
>                                 | flush_to_ldisc()                        |
>                                 |  grab spin lock                         |
> tty_buffer_flush()              |                                         | flush_to_ldisc()
>  wait for spin lock             |                                         |  wait for spin lock
>                                 |   if (!test_and_set_bit(TTYP_FLUSHING)) |
>                                 |    while (next flip buffer)             |
>                                 |     ...                                 |
>                                 |     drop spin lock                      |
>   grab spin lock                |                                         |
>    if (test_bit(TTYP_FLUSHING)) |                                         |
>     set_bit(TTYP_FLUSHPENDING)  |      receive_buf()                      |
>     drop spin lock              |                                         |
>                                 |                                         |   grab spin lock
>                                 |                                         |    if (!test_and_set_bit(TTYP_FLUSHING))
>                                 |                                         |    if (test_bit(TTYP_FLUSHPENDING))
>                                 |                                         |    __tty_buffer_flush()
> 
> CPU 2 has just flushed and freed all tty flip buffers while CPU 1 is
> transferring data from the head flip buffer.
> 
> The original patch was rejected under the assumption that parallel
> flush_to_ldisc() was not possible. Because of necessary changes to
> the workqueue api, work items can execute in parallel on SMP.
> 
> This patch differs slightly from the original patch by testing for
> a pending flush _after_ each receive_buf(), since TTYP_FLUSHPENDING
> can only be set while the lock is dropped around receive_buf().
> 
> Reported-by: Ilya Zykov <linux@...k.ru>
> Signed-off-by: Peter Hurley <peter@...leysoftware.com>
> ---
>  drivers/tty/tty_buffer.c | 22 ++++++++++------------
>  1 file changed, 10 insertions(+), 12 deletions(-)

What tree is this against?  Does it need your other larger set of ldisc
patches, or can it be applied without it?

confused,

greg k-h
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