linux-kernel - Aw: [PATCH 0/6] hv_netvsc: avoid races on mtu change/set channels

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Date:	Thu, 12 May 2016 16:58:32 +0200
From:	"Lino Sanfilippo" <LinoSanfilippo@....de>
To:	"Vitaly Kuznetsov" <vkuznets@...hat.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	devel@...uxdriverproject.org,
	"Haiyang Zhang" <haiyangz@...rosoft.com>,
	"K. Y. Srinivasan" <kys@...rosoft.com>
Subject: Aw: [PATCH 0/6] hv_netvsc: avoid races on mtu change/set channels

Hi,

>
> MTU change and set channels operations are implemented as netvsc device
> re-creation destroying internal structures (struct net_device stays). This
> is really unfortunate but there is no support from Hyper-V host to do it
> in a different way. Such re-creation is unsurprisingly racy, Haiyang
> reported a crash when netvsc_change_mtu() is racing with
> netvsc_link_change() but I was able to identify additional races upon
> investigation. Both netvsc_set_channels() and netvsc_change_mtu() race
> against:
> 1) netvsc_link_change()
> 2) netvsc_remove()
> 3) netvsc_send()
> 

after having a look into this driver I got the impression that you are working around an
unfortunate implementation of the shutdown sequence in the remove function:
If you do unregister_netdev() first instead of resource cleanup then neither set_channels()
nor change_mtu() can race with remove(). This is since after unregister_netdev() returns
the netdev is not longer available from userspace and thus neither set_channels nor
change_mtu can be called anymore (note that all of these functions are protected by the 
rtnl_lock).

To avoid the race between netvsc_change_mtu()/netvsc_set_channels() and netvsc_link_change()
you have to stop the concerning worker thread (dwork) before you call netvsc_close() and
restart it once the device is up again.

Regards,
Lino