netdev - Re: [PATCH] USB host CDC Phonet network interface driver

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Message-Id: <200907200935.19103.remi.denis-courmont@nokia.com>
Date:	Mon, 20 Jul 2009 09:35:18 +0300
From:	"Rémi Denis-Courmont" 
	<remi.denis-courmont@...ia.com>
To:	ext Oliver Neukum <oliver@...kum.org>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
Subject: Re: [PATCH] USB host CDC Phonet network interface driver

On Friday 17 July 2009 16:47:38 ext Oliver Neukum wrote:
> Am Freitag, 17. Juli 2009 11:56:06 schrieb Rémi Denis-Courmont:
> > +       atomic_dec(&pnd->tx_queue);
> > +       netif_wake_queue(dev);
>
> Now that I think about it this seems to be a race condition.
> What makes sure that your are still below the limit when you
> wake the queue?

The netif TX queue serializes usbpn_xmit() against itself. Then, at all times: 
tx_queue + !netif_queue_stopped() <= tx_queue_len (in other words either 
tx_queue = tx_queue_len and queue is stopped, or tx_queue < tx_queue_len).

Initially:
0 + 1 <= tx_queue_len, the assertion is initially true.

Recurrently:
If usbpn_xmit() is called, then the queue was not stopped, so tx_queue < 
tx_queue_len. Then the queue is stopped. In the race free cases, either 
tx_queue is incremented up to the limit, and the queue remains stopped; the 
assertion is still valid.
Alternatively tx_queue remains below limit and the queue gets woken again; the 
assertion is still valid.

In the racy case, tx_complete() fires, incrementation in usbpn_xmit() and 
decrementation in tx_complete() will cancel each other. So, regardless of 
their respective order, tx_queue will be unchanged, and the assertion remains 
valid. A fortiori, it works fine if usbpn_xmit() races with more than one call 
of tx_complete().

The cases that tx_complete() runs while the queue is not transmitting is 
evident.

-- 
Rémi Denis-Courmont
Nokia Devices R&D, Maemo Software, Helsinki

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