linux-kernel - Re: [PATCH v2 1/3] can: kvaser_usb: Fix tx queue start/stop race conditions

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Message-ID: <55006114.8030702@pengutronix.de>
Date:	Wed, 11 Mar 2015 16:36:52 +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>
Subject: Re: [PATCH v2 1/3] can: kvaser_usb: Fix tx queue start/stop
 race conditions

On 03/11/2015 04:23 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.
> 
> start_xmit()                             tx_acknowledge()
> ............                             ................
> atomic_inc(&tx_urbs);
> if (atomic_read(&tx_urbs) >= 16) {
>                         URB completion IRQ!
>                         -->
>                                          atomic_dec(&tx_urbs);
>                                          netif_wake_queue();
>                                          return;
>                         <--
>                         end of IRQ!
>     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 protection.

I'm missing a spin_lock_init(), right? Please compile and test your code
with everything switch on in Kernel hacking -> Lock Debugging.

Marc

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