netdev - hv_netvsc: WARNING: suspicious RCU usage?

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Date:   Mon, 5 Aug 2019 23:55:47 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Yidong Ren <Yidong.Ren@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "David S. Miller" <davem@...emloft.net>
CC:     "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
Subject: hv_netvsc: WARNING: suspicious RCU usage?

Hi,
After the VM boots up, I always get the below call-trace when I run "nload" for the first time:

[  113.910911] WARNING: suspicious RCU usage
[  113.913244] 5.2.0+ #19 Not tainted
[  113.915216] -----------------------------
[  113.917521] drivers/net/hyperv/netvsc_drv.c:1243 suspicious rcu_dereference_check() usage!
[  113.922191]
[  113.922191] other info that might help us debug this:
[  113.926573]
[  113.926573] rcu_scheduler_active = 2, debug_locks = 1
[  113.930052] 4 locks held by nload/1977:
[  113.932251]  #0: 0000000080b71e86 (&p->lock){+.+.}, at: seq_read+0x41/0x3d0
[  113.936115]  #1: 00000000cacff770 (&of->mutex){+.+.}, at: kernfs_seq_start+0x2a/0x90
[  113.940115]  #2: 00000000287c988f (kn->count#134){.+.+}, at: kernfs_seq_start+0x32/0x90
[  113.944292]  #3: 00000000996fa9cc (dev_base_lock){++.+}, at: netstat_show.isra.25+0x4a/0xb0
[  113.958076]
[  113.958076] stack backtrace:
[  113.958081] CPU: 3 PID: 1977 Comm: nload Not tainted 5.2.0+ #19
[  113.958083] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS 090006  04/28/2016
[  113.958084] Call Trace:
[  113.958091]  dump_stack+0x67/0x90
[  113.973663]  netvsc_get_stats64+0x159/0x170 [hv_netvsc]
[  113.973663]  dev_get_stats+0x55/0xb0
[  113.973663]  netstat_show.isra.25+0x5b/0xb0
[  113.973663]  dev_attr_show+0x15/0x40
[  113.981661]  sysfs_kf_seq_show+0xad/0xf0
[  113.981661]  seq_read+0x146/0x3d0
[  113.981661]  vfs_read+0x9c/0x160
[  113.989025]  ksys_read+0x5c/0xd0
[  113.989025]  do_syscall_64+0x5e/0x220
[  113.989025]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  113.989025] RIP: 0033:0x7f4485daaf31

nload is a console application which monitors network traffic and bandwidth usage in real time.

The warning is caused by the rcu_dereference_rtnl() :

1239 static void netvsc_get_stats64(struct net_device *net,
1240                                struct rtnl_link_stats64 *t)
1241 {
1242         struct net_device_context *ndev_ctx = netdev_priv(net);
1243         struct netvsc_device *nvdev = rcu_dereference_rtnl(ndev_ctx->nvdev);

I think here netvsc_get_stats64() neither holds rcu_read_lock() nor RTNL

IMO it should call rcu_read_lock()/unlock(), or get RTNL to fix the warning?

Thanks,
-- Dexuan