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

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Message-ID: <87r3d7mhy3.fsf@vitty.brq.redhat.com>
Date:	Thu, 12 May 2016 17:09:24 +0200
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	"Lino Sanfilippo" <LinoSanfilippo@....de>
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: Re: Aw: [PATCH 0/6] hv_netvsc: avoid races on mtu change/set channels

"Lino Sanfilippo" <LinoSanfilippo@....de> writes:

> 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).

It's worse: before the patch series we get 'struct hv_device' (as it is
called from core VMBus code and we simply cannot get to 'struct
net_device' we need without traveling through 'struct
netvsc_device'. This structure is removed and re-created by both
netvsc_set_channels() and netvsc_change_mtu().

>
> 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.

Yes, but we also need to guarantee this won't get rescheduled so we need
a flag for that. The appropriate flag is start_remove but only after we
move it from a structure we remove.

-- 
  Vitaly