From: Vitaly Kuznetsov <vkuznets@redhat•com>
To: Stephen Hemminger <stephen@networkplumber•org>
Cc: netdev@vger•kernel.org, Haiyang Zhang <haiyangz@microsoft•com>,
Stephen Hemminger <sthemmin@microsoft•com>,
linux-kernel@vger•kernel.org, devel@linuxdriverproject•org
Subject: Re: [PATCH net-next 2/4] hv_netvsc: protect nvdev->extension with RCU
Date: Tue, 31 Oct 2017 18:13:58 +0100 [thread overview]
Message-ID: <87shdzw889.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20171031174451.096978cd@shemminger-XPS-13-9360> (Stephen Hemminger's message of "Tue, 31 Oct 2017 17:44:51 +0100")
Stephen Hemminger <stephen@networkplumber•org> writes:
> On Tue, 31 Oct 2017 14:42:02 +0100
> Vitaly Kuznetsov <vkuznets@redhat•com> wrote:
>
>> @@ -2002,7 +2002,9 @@ static int netvsc_probe(struct hv_device *dev,
>> device_info.recv_sections = NETVSC_DEFAULT_RX;
>> device_info.recv_section_size = NETVSC_RECV_SECTION_SIZE;
>>
>> + rtnl_lock();
>> nvdev = rndis_filter_device_add(dev, &device_info);
>> + rtnl_unlock();
>
> rtnl is not necessary here. probe can not be bothered by other changes.
>
Yes, this is only to support rtnl_dereference() down the stack.
>> --- a/drivers/net/hyperv/rndis_filter.c
>> +++ b/drivers/net/hyperv/rndis_filter.c
>> @@ -402,20 +402,27 @@ int rndis_filter_receive(struct net_device *ndev,
>> void *data, u32 buflen)
>> {
>> struct net_device_context *net_device_ctx = netdev_priv(ndev);
>> - struct rndis_device *rndis_dev = net_dev->extension;
>> + struct rndis_device *rndis_dev;
>> struct rndis_message *rndis_msg = data;
>> + int ret = 0;
>> +
>> + rcu_read_lock_bh();
>> +
>> + rndis_dev = rcu_dereference_bh(net_dev->extension);
>
> filter_receive is already called only from NAPI only and has RCU lock and soft
> irq disabled. This is not necessary.
>
>> - net_dev->extension = NULL;
>> + rcu_assign_pointer(net_dev->extension, NULL);
>> +
>> + synchronize_rcu();
>
> rcu_assign_pointer with NULL is never a good idea.
> And synchronize_rcu is slow. Since net_device is already protected
> by RCU (for deletion) it should not be necessary.
>
I thought we don't care that much about the speed of this patch as
rndis_filter_device_remove() is only called on device remove/mtu
change/... and we need to interact with the host -- and this is already
slow.
> Thank you for trying to address these races. But it should be
> done carefully not by just slapping RCU everywhere.
Ok, I may have missed something. I'll try reproducing the crash and
finding a better fine-grained solution.
Thanks for the feedback!
--
Vitaly
next prev parent reply other threads:[~2017-10-31 17:13 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-31 13:42 [PATCH net-next 0/4] hv_netvsc: fix some crashes and hangs on channel/mtu changes Vitaly Kuznetsov
2017-10-31 13:42 ` [PATCH net-next 1/4] hv_netvsc: netvsc_teardown_gpadl() split Vitaly Kuznetsov
2017-10-31 16:37 ` Stephen Hemminger
2017-10-31 13:42 ` [PATCH net-next 2/4] hv_netvsc: protect nvdev->extension with RCU Vitaly Kuznetsov
2017-10-31 16:44 ` Stephen Hemminger
2017-10-31 17:13 ` Vitaly Kuznetsov [this message]
2017-11-02 10:27 ` Vitaly Kuznetsov
2017-10-31 13:42 ` [PATCH net-next 3/4] hv_netvsc: reset net_device_ctx->nvdev with rcu_assign_pointer() Vitaly Kuznetsov
2017-10-31 14:09 ` Eric Dumazet
2017-10-31 14:40 ` Vitaly Kuznetsov
2017-10-31 14:44 ` David Miller
2017-10-31 14:50 ` Vitaly Kuznetsov
2017-10-31 16:45 ` Stephen Hemminger
2017-10-31 13:42 ` [PATCH net-next 4/4] hv_netvsc: hide warnings about uninitialized/missing rndis device Vitaly Kuznetsov
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