Bug 1294521

Description of problem:
I had the following setup:
1GB btrfs image stored in tmpfs, bound to a loop device and mounted, accessed through an overlayfs mount with two different subvolumes as lowerdir and upperdir, the former read-only, the latter writable. The workdir was also a directory in the btrfs mount, in the root subvolume.

When I tried to change a file with vi, the system crashed.

Additional info:
reporter:       libreport-2.6.3
BUG: unable to handle kernel NULL pointer dereference at 0000000000000334
IP: [<ffffffffa0c594bc>] btrfs_sync_file+0xcc/0x350 [btrfs]
PGD 0 
Oops: 0002 [#1] SMP 
Modules linked in: btrfs xor raid6_pq nls_utf8 hfsplus hfs isofs loop xt_CHECKSUM tun veth ipt_MASQUERADE nf_nat_masquerade_ipv4 xt_addrtype br_netfilter overlay rfcomm fuse ccm nf_conntrack_netbios_ns nf_conntrack_broadcast ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 xt_conntrack ebtable_filter ebtable_nat ebtable_broute bridge ebtables ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_raw ip6table_mangle ip6table_security ip6table_filter ip6_tables iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_raw iptable_mangle iptable_security bnep vfat fat uvcvideo videobuf2_vmalloc videobuf2_core iTCO_wdt iTCO_vendor_support snd_hda_codec_realtek arc4 videobuf2_memops acer_wmi ath9k sparse_keymap v4l2_common ath9k_common intel_rapl ath9k_hw iosf_mbi
 snd_hda_codec_hdmi snd_hda_codec_generic snd_soc_rt5640 ath3k btusb btrtl videodev ath snd_soc_rl6231 x86_pkg_temp_thermal btbcm mac80211 coretemp snd_hda_intel snd_hda_codec snd_soc_core kvm_intel media btintel snd_hda_core snd_hwdep snd_compress bluetooth cfg80211 snd_pcm_dmaengine kvm crct10dif_pclmul crc32_pclmul ac97_bus crc32c_intel snd_seq snd_seq_device rfkill snd_pcm snd_timer joydev snd mei_me mei dw_dmac shpchp dw_dmac_core lpc_ich i2c_designware_platform snd_soc_sst_acpi soundcore soc_button_array i2c_designware_core i2c_i801 wmi nfsd auth_rpcgss nfs_acl lockd grace sunrpc binfmt_misc amdkfd amd_iommu_v2 radeon i915 8021q garp stp llc mrp ttm i2c_algo_bit drm_kms_helper serio_raw drm tg3 sdhci_pci ptp pps_core sdhci_acpi sdhci mmc_core video i2c_hid
CPU: 2 PID: 11227 Comm: vi Not tainted 4.2.8-300.fc23.x86_64 #1
Hardware name: Acer Aspire E1-572G/EA50_HW   , BIOS V2.14 01/15/2014
task: ffff880219abd880 ti: ffff8801e182c000 task.ti: ffff8801e182c000
RIP: 0010:[<ffffffffa0c594bc>]  [<ffffffffa0c594bc>] btrfs_sync_file+0xcc/0x350 [btrfs]
RSP: 0018:ffff8801e182fe28  EFLAGS: 00010292
RAX: ffff880219abd880 RBX: ffff8801a91b3b00 RCX: 0000000000000000
RDX: 0000000080000000 RSI: 0000000000000000 RDI: ffff88022a5b6660
RBP: ffff8801e182feb8 R08: 0000000000000001 R09: 0000000000000f00
R10: 0000000000000000 R11: 0000000000000246 R12: ffff88022a5b6660
R13: ffff88022a5b65b8 R14: 0000000000000000 R15: 0000000000000000
FS:  00007f98e5a8a800(0000) GS:ffff88025f280000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000334 CR3: 0000000214cfa000 CR4: 00000000001426e0
Stack:
 ffff8801a91b3b10 0000000200000001 8000000000000000 ffff88024c175000
 7fffffffffffffff 0000000000000000 ffff8801a91b3b10 0000000000000000
 0000000000000000 ffff880230a23000 0000000000001000 00000000d289e66e
Call Trace:
 [<ffffffff8124fd39>] vfs_fsync_range+0x49/0xa0
 [<ffffffff8124fded>] do_fsync+0x3d/0x70
 [<ffffffff8121df42>] ? SyS_lseek+0x92/0xb0
 [<ffffffff81250090>] SyS_fsync+0x10/0x20
 [<ffffffff8177a2ae>] entry_SYSCALL_64_fastpath+0x12/0x71
Code: 8b 0e 48 85 c9 75 e8 eb 9e 48 8b 45 90 4c 8b 75 98 4d 8d a5 a8 00 00 00 4c 89 e7 4c 29 f0 48 83 c0 01 48 89 45 80 e8 c4 ea b1 e0 <f0> 41 ff 87 34 03 00 00 49 8b 85 70 ff ff ff 48 c1 e8 07 83 e0 
RIP  [<ffffffffa0c594bc>] btrfs_sync_file+0xcc/0x350 [btrfs]
 RSP <ffff8801e182fe28>
CR2: 0000000000000334