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

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Message-ID: <20150311155736.GA12396@linux>
Date:	Wed, 11 Mar 2015 11:57:36 -0400
From:	"Ahmed S. Darwish" <darwish.07@...il.com>
To:	Marc Kleine-Budde <mkl@...gutronix.de>
Cc:	Olivier Sobrie <olivier@...rie.be>,
	Oliver Hartkopp <socketcan@...tkopp.net>,
	Wolfgang Grandegger <wg@...ndegger.com>,
	Andri Yngvason <andri.yngvason@...el.com>,
	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 Wed, Mar 11, 2015 at 04:36:52PM +0100, Marc Kleine-Budde wrote:
> 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.
> 

Ouch... that passed through it seems since __ARCH_SPIN_LOCK_UNLOCKED
is always zero on x86. Recompiling the kernel and re-iterating another
patch series.

Thanks,
Darwish
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