Date: Tue, 22 Mar 2011 18:05:45 +0000
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Toby Gray <toby.gray@realvnc.com>
Cc: Johan Hovold <jhovold@gmail.com>, Oliver Neukum <oliver@neukum.name>,
        Greg Kroah-Hartman <gregkh@suse.de>, linux-usb@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] USB: cdc-acm: Prevent data loss when filling tty
 buffer.
Message-ID: <20110322180545.54df2fa0@lxorguk.ukuu.org.uk>
In-Reply-To: <4D88ADFB.10206@realvnc.com>
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> Is there a mechanism preventing a different thread from running 
> n_tty_read between n_tty_receive_buf finding receive_room to be below 
> the threshold and tty_throttle being called? If not then isn't there a 
> race condition when the following happens:

n_tty_receive is single threaded and is going to get run in preference to
user threads.

>          1. n_tty_receive_buf fills up the buffer so that the free space 
> is below TTY_THRESHOLD_THROTTLE
>          2. n_tty_receive_buf comes to the check at the end and decide 
> that it needs to call tty_throttle
>          3. Thread rescheduling happens and a different thread runs 
> n_tty_read which empties the buffer
>          4. After emptying the buffer n_tty_read calls tty_unthrottle, 
> which does nothing as the throttling bit isn't set
>          5. The n_tty_receive_buf thread is executed again, calling 
> tty_throttle, causing throttling, but with an empty buffer.
> 
> Or have I not understood a complexity in the interactions within n_tty.c?

Looks possible - historically it would have been safe but not any more.
The scenario I think would have to be two thread of execution in parallel
on two processors at the same moment and with near perfect timing but I
don't see why it can't happen.

Alan
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