linux-kernel - RE: [PATCH] xen-netfront: set real_num_tx_queues to zreo avoid to trigger BUG

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Date:	Sat, 20 Feb 2016 06:00:22 +0000
From:	"Gonglei (Arei)" <arei.gonglei@...wei.com>
To:	David Miller <davem@...emloft.net>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Huangpeng (Peter)" <peter.huangpeng@...wei.com>
Subject: RE: [PATCH] xen-netfront: set real_num_tx_queues to zreo avoid to
 trigger BUG_ON

Hi,

Thanks for rapid feedback :)

> From: David Miller [mailto:davem@...emloft.net]
> Sent: Saturday, February 20, 2016 12:37 PM
> 
> From: Gonglei <arei.gonglei@...wei.com>
> Date: Sat, 20 Feb 2016 09:27:26 +0800
> 
> > It's possible for a race condition to exist between xennet_open() and
> > talk_to_netback(). After invoking netfront_probe() then other
> > threads or processes invoke xennet_open (such as NetworkManager)
> > immediately may trigger BUG_ON(). Besides, we also should reset
> > real_num_tx_queues in xennet_destroy_queues().
> 
> One should really never invoke register_netdev() until the device is
> %100 fully initialized.
> 
> This means you cannot call register_netdev() until it is completely
> legal to invoke your ->open() method.
> 
> And I think that is what the real problem is here.
> 
> If you follow the correct rules for ordering wrt. register_netdev()
> there are no "races".  Because ->open() must be legally invokable
> from the exact moment you call register_netdev().
> 

Yes, I agree. Though that's the historic legacy problem. ;)

> I'm not applying this, as it really sounds like the fundamental issue
> is the order in which the xen-netfront private data is initialized
> or setup before being registered.

That means register_netdev() should be invoked after xennet_connect(), right?

Regards,
-Gonglei