Re: [PATCH] tty_buffer: avoid race due to tty_buffer_free_all() being misused

[channing <chao.bi@intel.com>]

On Thu, 2013-05-16 at 08:54 -0400, Peter Hurley wrote:
> On 05/16/2013 04:59 AM, channing wrote:
> >
> > In tty_buffer.c, function tty_buffer_free_all() is used to remove
> > all buffers for a tty, although it's declared that it mustn't be called
> > when the tty is in use, it cannot guarantee that. we can observe some
> > device driver make use it by mistake, for example, while tty device is
> > releasing, the tty data forwarding is not stopped, then it might hit
> > the case that tty buffer is being used while tty_buffer_free_all()
> > free this tty buffer, and finally lead to random error at any places,
> > and it's not clear to debug.
> 
> What kernel version?
3.4
> 
> > Although device driver could do better, it's simpler and safer to
> > strengthen protection in the view of tty buffer, by adding a tty->buf.lock
> > in tty_buffer_free_all() to avoid it racing with ongoing tty buffer
> > operations.
> 
> Sorry, but this isn't correct.
> 
> The driver cannot continue to perform i/o concurrently with
> tty_port_destroy().
> 
Thanks for remind, 3.4 haven't tty_port_destroy(), the mainline has
changed the way to call tty_buffer_free_all().

> If the concurrent use you're observing is with flush_to_ldisc(),
> that should be fixed in current mainline.
> 
Yes, when calling flush_to_ldisc() in Kernel 3.4, we could observe the
tty buffer is corrupted, and dummped stack shows that
tty_buffer_free_all() was called before. Is it a known issue fixed in
old version? would you please tell me related patch to solve this
flush_to_ldisc() issue? Thanks very much.

Chao