netdev - Re: [PATCH v4 1/3] can: kvaser_usb: Fix tx queue start/stop race conditions

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Message-ID: <55043A7E.7040001@pengutronix.de>
Date:	Sat, 14 Mar 2015 14:41:18 +0100
From:	Marc Kleine-Budde <mkl@...gutronix.de>
To:	"Ahmed S. Darwish" <darwish.07@...il.com>,
	Olivier Sobrie <olivier@...rie.be>,
	Oliver Hartkopp <socketcan@...tkopp.net>,
	Wolfgang Grandegger <wg@...ndegger.com>,
	Andri Yngvason <andri.yngvason@...el.com>
CC:	Linux-CAN <linux-can@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH v4 1/3] can: kvaser_usb: Fix tx queue start/stop
 race conditions

On 03/14/2015 02:02 PM, Ahmed S. Darwish wrote:
> From: Ahmed S. Darwish <ahmed.darwish@...eo.com>
> 
> A number of tx queue wake-up events went missing due to the
> outlined scenario below. Start state is a pool of 16 tx URBs,
> active tx_urbs count = 15, with the netdev tx queue open.
> 
> CPU #1 [softirq]                         CPU #2 [softirq]
> start_xmit()                             tx_acknowledge()
> ................                         ................
> 
> atomic_inc(&tx_urbs);
> if (atomic_read(&tx_urbs) >= 16) {
>                         -->
>                                          atomic_dec(&tx_urbs);
>                                          netif_wake_queue();
>                                          return;
>                         <--
>     netif_stop_queue();
> }
> 
> At the end, the correct state expected is a 15 tx_urbs count
> value with the tx queue state _open_. Due to the race, we get
> the same tx_urbs value but with the tx queue state _stopped_.
> The wake-up event is completely lost.
> 
> Thus avoid hand-rolled concurrency mechanisms and use a proper
> lock for contexts and tx queue protection.
> 
> Signed-off-by: Ahmed S. Darwish <ahmed.darwish@...eo.com>

Applied to can. This will go into David's net tree and finally into
net-next. Then I'll apply patches 2+3. Nag me, if I forget about them ;)

Thanks,
Marc

-- 
Pengutronix e.K.                  | Marc Kleine-Budde           |
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