yet another call trace during send/receive

Hi.

On Debian sid:

$ uname -a
Linux heisenberg 4.8.0-2-amd64 #1 SMP Debian 4.8.15-2 (2017-01-04) x86_64 GNU/Linux

$ btrfs version
btrfs-progs v4.7.3

During a:
# btrfs send -p foo bar | btrfs receive baz

Jan 11 20:43:10 heisenberg kernel: ------------[ cut here ]------------
Jan 11 20:43:10 heisenberg kernel: WARNING: CPU: 6 PID: 10042 at /build/linux-zDY19G/linux-4.8.15/fs/btrfs/send.c:6117 btrfs_ioctl_send+0x533/0x1280 [btrfs]
Jan 11 20:43:10 heisenberg kernel: Modules linked in: udp_diag tcp_diag inet_diag algif_skcipher af_alg uas vhost_net vhost macvtap macvlan xt_CHECKSUM iptable_mangle ipt_MASQUERADE nf_nat_masquerade_ipv4 iptable_nat nf_nat_ipv4 nf_nat tun bridge stp llc fuse ctr ccm ebtable_filter ebtables joydev rtsx_pci_ms memstick rtsx_pci_sdmmc mmc_core iTCO_wdt iTCO_vendor_support cpufreq_userspace cpufreq_powersave cpufreq_conservative ip6t_REJECT nf_reject_ipv6 xt_tcpudp nf_conntrack_ipv6 nf_defrag_ipv6 ip6table_filter ip6_tables xt_policy ipt_REJECT nf_reject_ipv4 xt_comment nf_conntrack_ipv4 nf_defrag_ipv4 xt_multiport xt_conntrack nf_conntrack iptable_filter binfmt_misc intel_rapl x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel kvm irqbypass crct10dif_pclmul crc32_pclmul ghash_clmulni_intel intel_cstate intel_uncore
Jan 11 20:43:10 heisenberg kernel:  intel_rapl_perf psmouse pcspkr uvcvideo videobuf2_vmalloc videobuf2_memops videobuf2_v4l2 videobuf2_core videodev media btusb btrtl btbcm btintel sg bluetooth crc16 arc4 iwldvm mac80211 iwlwifi cfg80211 rtsx_pci rfkill fjes snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic tpm_tis tpm_tis_core tpm i915 fujitsu_laptop battery snd_hda_intel snd_hda_codec lpc_ich i2c_i801 ac mfd_core shpchp i2c_smbus snd_hda_core snd_hwdep snd_pcm snd_timer e1000e snd soundcore ptp pps_core video button mei_me mei drm_kms_helper drm i2c_algo_bit loop parport_pc ppdev sunrpc lp parport ip_tables x_tables autofs4 dm_crypt dm_mod raid10 raid456 libcrc32c async_raid6_recov async_memcpy async_pq async_xor async_tx raid1 raid0 multipath linear md_mod btrfs crc32c_generic xor raid6_pq uhci_hcd usb_storage
Jan 11 20:43:10 heisenberg kernel:  sd_mod crc32c_intel ahci libahci aesni_intel xhci_pci aes_x86_64 xhci_hcd libata glue_helper lrw ehci_pci gf128mul ablk_helper ehci_hcd cryptd evdev usbcore scsi_mod serio_raw usb_common
Jan 11 20:43:10 heisenberg kernel: CPU: 6 PID: 10042 Comm: btrfs Tainted: G        W       4.8.0-2-amd64 #1 Debian 4.8.15-2
Jan 11 20:43:10 heisenberg kernel: Hardware name: FUJITSU LIFEBOOK E782/FJNB23E, BIOS Version 1.11 05/24/2012
Jan 11 20:43:10 heisenberg kernel:  0000000000000286 00000000248adbdb ffffffffb3b1f925 0000000000000000
Jan 11 20:43:10 heisenberg kernel:  0000000000000000 ffffffffb3874ffe ffff9ebe7e9f4424 00007ffcbf0ea5d0
Jan 11 20:43:10 heisenberg kernel:  ffff9ebc0d644000 ffff9ebe7e9f4000 ffff9ebe5e44fb20 ffff9ebd4270ae00
Jan 11 20:43:10 heisenberg kernel: Call Trace:
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffb3b1f925>] ? dump_stack+0x5c/0x77
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffb3874ffe>] ? __warn+0xbe/0xe0
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffc034a533>] ? btrfs_ioctl_send+0x533/0x1280 [btrfs]
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffb39f1500>] ? memcg_kmem_get_cache+0x50/0x150
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffb39da242>] ? kmem_cache_alloc+0x122/0x530
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffb38a4bf1>] ? sched_slice.isra.57+0x51/0xc0
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffb38a4f10>] ? update_cfs_rq_load_avg+0x200/0x4c0
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffb389bc96>] ? task_rq_lock+0x46/0xa0
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffc030f9fc>] ? btrfs_ioctl+0x97c/0x2370 [btrfs]
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffb38aa7bc>] ? enqueue_task_fair+0x5c/0x940
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffb382e8a5>] ? sched_clock+0x5/0x10
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffb389d5b0>] ? check_preempt_curr+0x50/0x90
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffb389f366>] ? wake_up_new_task+0x156/0x200
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffb3a0ebef>] ? do_vfs_ioctl+0x9f/0x5f0
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffb38742bd>] ? _do_fork+0x14d/0x3f0
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffb3a0f1b4>] ? SyS_ioctl+0x74/0x80
Jan 11 20:43:10 heisenberg kernel:  [<ffffffffb3de8cb6>] ? system_call_fast_compare_end+0xc/0x96
Jan 11 20:43:10 heisenberg kernel: ---[ end trace 3831b8afbd0cbc9e ]---
Jan 11 20:43:45 heisenberg kernel: BTRFS info (device dm-2): The free space cache file (7525348933632) is invalid. skip it

The send/receive seems to continue running...
Not sure if the free space cache file entry is related (btw: a btrfs check directly before didn't find that error - actually yet another fsck directly before that, brought a message that the super generation and space file generation would mismatch (or something like that) and it would be invalidated... so kinda strange that this happens at all).

Cheers,
Chris.