netdev - Re: [PATCH] xen-netfront: set real_num_tx_queues to zreo avoid to trigger BUG

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Message-Id: <20160219.233643.95383657086552033.davem@davemloft.net>
Date:	Fri, 19 Feb 2016 23:36:43 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	arei.gonglei@...wei.com
Cc:	netdev@...r.kernel.org, xen-devel@...ts.xenproject.org,
	linux-kernel@...r.kernel.org, peter.huangpeng@...wei.com
Subject: Re: [PATCH] xen-netfront: set real_num_tx_queues to zreo avoid to
 trigger BUG_ON

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().

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.