net/sunrpc/clnt.c:2773 suspicious rcu_dereference_check() usage!

[Ross Zwisler <ross.zwisler@linux.intel.com>]

I've got a virtual machine that has some NFS mounts, and with a newly compiled
kernel based on v4.9-rc3 I see the following warning/info message:

[   42.750181] ===============================
[   42.750192] [ INFO: suspicious RCU usage. ]
[   42.750203] 4.9.0-rc3-00002-g7b6e7de #3 Not tainted
[   42.750213] -------------------------------
[   42.750225] net/sunrpc/clnt.c:2773 suspicious rcu_dereference_check() usage!
[   42.750235] 
[   42.750235] other info that might help us debug this:
[   42.750235] 
[   42.750246] 
[   42.750246] rcu_scheduler_active = 1, debug_locks = 0
[   42.750257] 1 lock held by mount.nfs4/6440:
[   42.750278]  #0: 
[   42.750299]  (
[   42.750319] &(&nn->nfs_client_lock)->rlock
[   42.750340] ){+.+...}
[   42.750362] , at: 
[   42.750372] [<ffffffff813012b5>] nfs_get_client+0x105/0x5e0
[   42.750383] 
[   42.750383] stack backtrace:
[   42.750394] CPU: 0 PID: 6440 Comm: mount.nfs4 Not tainted 4.9.0-rc3-00002-g7b6e7de #3
[   42.750406] Hardware name: Intel Corporation PURLEY/PURLEY, BIOS PLYDCRB1.MBH.0096.D23.1608240105 08/24/2016
[   42.750429]  ffffc9000092fa68 ffffffff8150730f ffff88014ec8da40 0000000000000001
[   42.750452]  ffffc9000092fa98 ffffffff810bc3f7 ffff880150b0b228 ffff88015068dbb0
[   42.750475]  ffffc9000092fb38 ffff88014fc99180 ffffc9000092fac0 ffffffff81b243e5
[   42.750486] Call Trace:
[   42.750498]  [<ffffffff8150730f>] dump_stack+0x67/0x98
[   42.750511]  [<ffffffff810bc3f7>] lockdep_rcu_suspicious+0xe7/0x120
[   42.750524]  [<ffffffff81b243e5>] rpc_clnt_xprt_switch_has_addr+0x115/0x150
[   42.750536]  [<ffffffff813013f4>] nfs_get_client+0x244/0x5e0
[   42.750549]  [<ffffffff813012ac>] ? nfs_get_client+0xfc/0x5e0
[   42.750561]  [<ffffffff813568f8>] nfs4_set_client+0x98/0x130
[   42.750574]  [<ffffffff8135872e>] nfs4_create_server+0x13e/0x390
[   42.750588]  [<ffffffff8134cd0e>] nfs4_remote_mount+0x2e/0x60
[   42.750600]  [<ffffffff811f3a29>] mount_fs+0x39/0x170
[   42.750614]  [<ffffffff81214a0b>] vfs_kern_mount+0x6b/0x150
[   42.750626]  [<ffffffff8134cbec>] ? nfs_do_root_mount+0x3c/0xc0
[   42.750639]  [<ffffffff8134cc36>] nfs_do_root_mount+0x86/0xc0
[   42.750652]  [<ffffffff8134d014>] nfs4_try_mount+0x44/0xc0
[   42.750664]  [<ffffffff81302097>] ? get_nfs_version+0x27/0x90
[   42.750677]  [<ffffffff81310f8c>] nfs_fs_mount+0x4ac/0xd80
[   42.750689]  [<ffffffff810bb938>] ? lockdep_init_map+0x88/0x1f0
[   42.750701]  [<ffffffff81311ac0>] ? nfs_clone_super+0x130/0x130
[   42.750713]  [<ffffffff8130f300>] ? param_set_portnr+0x70/0x70
[   42.750726]  [<ffffffff811f3a29>] mount_fs+0x39/0x170
[   42.750740]  [<ffffffff81214a0b>] vfs_kern_mount+0x6b/0x150
[   42.750752]  [<ffffffff812176f1>] do_mount+0x1f1/0xd10
[   42.750765]  [<ffffffff81217441>] ? copy_mount_options+0xa1/0x140
[   42.750777]  [<ffffffff81218543>] SyS_mount+0x83/0xd0
[   42.750790]  [<ffffffff81002abc>] do_syscall_64+0x5c/0x130
[   42.750802]  [<ffffffff81c479a4>] entry_SYSCALL64_slow_path+0x25/0x25

This rcu_dereference_check() was introduced by the following commit:

commit 39e5d2df959dd4aea81fa33d765d2a5cc67a0512
Author: Andy Adamson <andros@netapp.com>
Date:   Fri Sep 9 09:22:25 2016 -0400

    SUNRPC search xprt switch for sockaddr
    
    Signed-off-by: Andy Adamson <andros@netapp.com>
    Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

Thanks,
- Ross