RHEL7: kernel panics at nfs41_assign_slot due to NULL pointer dereference

Issue

• kernel panics at nfs41_assign_slot due to NULL pointer dereference
• The following messages and call trace are logged to kernel log at the time of the occurrence of the issue.

[21761.774192] BUG: unable to handle kernel NULL pointer dereference at 0000000000000014
[21761.774199] IP: [<ffffffffa04f5bed>] nfs41_assign_slot+0x3d/0x60 [nfsv4]
[21761.774218] PGD fd2b91067 PUD f12165067 PMD 0 
[21761.774221] Oops: 0002 [#1] SMP 
[21761.774223] Modules linked in: ip6table_filter ip6_tables iptable_filter ip_tables ebtable_nat ebtables sg nfsv3 rpcsec_gss_krb5 nfsv4 dns_resolver nfs fscache coretemp crct10dif_pclmul crc32_pclmul ppdev crc32c_intel ghash_clmulni_intel vmw_balloon aesni_intel ext4 lrw gf128mul glue_helper ablk_helper cryptd mbcache parport_pc parport jbd2 serio_raw pcspkr vmw_vmci shpchp i2c_piix4 mperf nfsd auth_rpcgss nfs_acl lockd sunrpc uinput xfs libcrc32c sr_mod cdrom ata_generic pata_acpi sd_mod crc_t10dif crct10dif_common vmwgfx ttm ata_piix drm libata vmxnet3 vmw_pvscsi i2c_core floppy dm_mirror dm_region_hash dm_log dm_mod
[21761.774260] CPU: 4 PID: 22640 Comm: kworker/4:1 Not tainted 3.10.0-123.8.1.el7.x86_64 #1
[21761.774262] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 06/22/2012
[21761.774274] Workqueue: rpciod rpc_async_schedule [sunrpc]
[21761.774276] task: ffff880fe4f316c0 ti: ffff8800b7766000 task.ti: ffff8800b7766000
[21761.774277] RIP: 0010:[<ffffffffa04f5bed>]  [<ffffffffa04f5bed>] nfs41_assign_slot+0x3d/0x60 [nfsv4]
[21761.774287] RSP: 0018:ffff8800b7767cc0  EFLAGS: 00010246
[21761.774288] RAX: 0000000101477b0a RBX: ffff880fe452ca98 RCX: ffff880fe452ca00
[21761.774290] RDX: 0000000000000000 RSI: ffff880fd78ae700 RDI: ffff880fe5ff1200
[21761.774291] RBP: ffff8800b7767cc0 R08: ffff880e5626d010 R09: 0000000000000001
[21761.774292] R10: dfc0206dbfc88688 R11: ffffea003f67a200 R12: ffff880fe5ff1200
[21761.774293] R13: ffffffffa04f5bb0 R14: ffff880fd78ae700 R15: ffff880fe452cab0
[21761.774295] FS:  0000000000000000(0000) GS:ffff88103fd00000(0000) knlGS:0000000000000000
[21761.774296] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[21761.774297] CR2: 0000000000000014 CR3: 0000000fc5887000 CR4: 00000000000407e0
[21761.774358] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[21761.774372] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[21761.774374] Stack:
[21761.774375]  ffff8800b7767d00 ffffffffa0296da4 ffff8800b7767d58 ffff880fd78ae700
[21761.774378]  ffff880fe53ea828 ffff880fe452ca00 ffff880fe452ca90 ffff880fd9e88600
[21761.774380]  ffff8800b7767d10 ffffffffa04f6322 ffff8800b7767d40 ffffffffa04d13b8
[21761.774383] Call Trace:
[21761.774394]  [<ffffffffa0296da4>] rpc_wake_up_first+0x64/0x1f0 [sunrpc]
[21761.774404]  [<ffffffffa04f6322>] nfs41_wake_and_assign_slot+0x32/0x40 [nfsv4]
[21761.774409]  [<ffffffffa04d13b8>] nfs40_sequence_done.isra.22+0x38/0x70 [nfsv4]
[21761.774417]  [<ffffffffa0295ad0>] ? rpc_destroy_wait_queue+0x20/0x20 [sunrpc]
[21761.774423]  [<ffffffffa04d17a8>] nfs4_sequence_done+0x38/0x40 [nfsv4]
[21761.774428]  [<ffffffffa04d3224>] nfs4_delegreturn_done+0x24/0x140 [nfsv4]
[21761.774435]  [<ffffffffa0295ad0>] ? rpc_destroy_wait_queue+0x20/0x20 [sunrpc]
[21761.774442]  [<ffffffffa0295ad0>] ? rpc_destroy_wait_queue+0x20/0x20 [sunrpc]
[21761.774449]  [<ffffffffa0295af8>] rpc_exit_task+0x28/0x90 [sunrpc]
[21761.774455]  [<ffffffffa0296834>] __rpc_execute+0x94/0x410 [sunrpc]
[21761.774462]  [<ffffffffa0296bd6>] rpc_async_schedule+0x26/0x30 [sunrpc]
[21761.774467]  [<ffffffff8107e03b>] process_one_work+0x17b/0x460
[21761.774470]  [<ffffffff8107ee0b>] worker_thread+0x11b/0x400
[21761.774472]  [<ffffffff8107ecf0>] ? rescuer_thread+0x400/0x400
[21761.774475]  [<ffffffff81085aff>] kthread+0xcf/0xe0
[21761.774478]  [<ffffffff81085a30>] ? kthread_create_on_node+0x140/0x140
[21761.774481]  [<ffffffff815f29ac>] ret_from_fork+0x7c/0xb0
[21761.774483]  [<ffffffff81085a30>] ? kthread_create_on_node+0x140/0x140
[21761.774484] Code: 89 e5 48 8b 81 a8 01 00 00 a8 01 74 09 31 c0 41 f6 40 08 02 74 2f 48 8b 81 80 01 00 00 48 89 46 10 49 89 30 48 8b 05 13 b4 4f e1 <c7> 42 14 00 00 00 00 48 89 32 c7 42 10 01 00 00 00 48 89 42 08 
[21761.774506] RIP  [<ffffffffa04f5bed>] nfs41_assign_slot+0x3d/0x60 [nfsv4]
[21761.774514]  RSP <ffff8800b7767cc0>
[21761.774515] CR2: 0000000000000014