exit_creds error in kthread_stop().

[   68.629247] BUG: unable to handle kernel NULL pointer dereference at           (null)
[   68.630173] IP: [] exit_creds+0x1f/0x70
[   68.630173] PGD 0 
[   68.630173] Oops: 0000 [#1] SMP 
[   68.630173] Modules linked in: btrfs(OE) ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 cfg80211 nf_conntrack_ipv6 nf_defrag_ipv6 ipt_REJECT nf_reject_ipv4 rfkill nf_conntrack_ipv4 nf_defrag_ipv4 xt_conntrack nf_conntrack ebtable_nat ebtable_broute bridge stp llc ebtable_filter ebtables ip6table_mangle ip6table_security ip6table_raw ip6table_filter ip6_tables iptable_mangle iptable_security iptable_raw iptable_filter ip_tables sg qxl snd_hda_codec_generic ext4 snd_hda_intel snd_hda_controller ttm snd_hda_codec drm_kms_helper snd_hwdep snd_seq drm snd_seq_device xor snd_pcm raid6_pq mbcache jbd2 snd_timer 8139too ppdev snd 8139cp parport_pc pcspkr i2c_piix4 serio_raw virtio_console nfsd soundcore parport virtio_balloon mii i2c_core auth_rpcgss nfs_acl lockd grace sunrpc xfs libcrc32c sd_mod ata_generic pata_acpi ata_piix virtio_pci virtio virtio_ring libata floppy dm_mirror dm_region_hash dm_log dm_mod [last unloaded: btrfs]
[   68.630173] CPU: 1 PID: 2629 Comm: mount Tainted: G           OE  3.19.0-rc1+ #55
[   68.630173] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[   68.630173] task: ffff88007c1c0d70 ti: ffff880079438000 task.ti: ffff880079438000
[   68.630173] RIP: 0010:[]  [] exit_creds+0x1f/0x70
[   68.630173] RSP: 0018:ffff88007943bb48  EFLAGS: 00010296
[   68.630173] RAX: 0000000000000000 RBX: ffff880077a14330 RCX: 0000000000000000
[   68.630173] RDX: ffff88006a02a000 RSI: ffff88003632b800 RDI: 0000000000000000
[   68.630173] RBP: ffff88007943bb58 R08: 000000000001dd20 R09: ffff88007fd1dd20
[   68.630173] R10: ffffffffa078149a R11: ffffea0001dae340 R12: ffff880077a14330
[   68.630173] R13: 0000000000000000 R14: ffff88006c74c1a0 R15: ffff880077c465e0
[   68.630173] FS:  00007f4f398ae880(0000) GS:ffff88007fd00000(0000) knlGS:0000000000000000
[   68.630173] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   68.630173] CR2: 0000000000000000 CR3: 0000000076f93000 CR4: 00000000000006e0
[   68.630173] Stack:
[   68.630173]  ffff88007943bb58 ffff880077a14330 ffff88007943bb78 ffffffff81073c12
[   68.630173]  0000000000000000 ffff880077a14330 ffff88007943bba8 ffffffff81095d50
[   68.630173]  ffff88007943bb98 ffff88006c7ac000 ffff88003632b800 ffff88006c7ac000
[   68.630173] Call Trace:
[   68.630173]  [] __put_task_struct+0x52/0x140
[   68.630173]  [] kthread_stop+0xf0/0x100