4.1.5: WARNING: CPU: 3 PID: 4756 at fs/btrfs/extent-tree.c:7539 btrfs_alloc_tree_block

Hello,

this is from vanilla 4.1.5:

[29948.701309] ------------[ cut here ]------------
[29948.756302] WARNING: CPU: 3 PID: 4756 at fs/btrfs/extent-tree.c:7539 btrfs_alloc_tree_block+0x431/0x4c0 [btrfs]()
[29948.814644] BTRFS: block rsv returned -28
[29948.814647] Modules linked in: dm_mod netconsole ipt_REJECT nf_reject_ipv4 xt_multiport iptable_filter ip_tables x_tables cpufreq_userspace cpufreq_stats cpufreq_powersave cpufreq_conservative bonding ext2 coretemp loop usbhid ehci_pci i2c_i801 ipmi_si ehci_hcd i2c_core usbcore shpchp usb_common ipmi_msghandler sb_edac edac_core button btrfs lzo_compress raid0 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq raid1 md_mod sg sd_mod i40e(O) ixgbe(O) vxlan ip6_udp_tunnel udp_tunnel ahci ptp aacraid libahci pps_core
[29949.135847] CPU: 3 PID: 4756 Comm: kworker/u65:6 Tainted: G        W 
 O    4.1.5+13-ph #1
[29949.203042] Hardware name: Supermicro X10DRH/X10DRH-IT, BIOS 1.0c 02/18/2015
[29949.270647] Workqueue: btrfs-extent-refs btrfs_extent_refs_helper [btrfs]
[29949.338876]  ffffffffa02783f3 ffff88068086f6c8 ffffffff8162dd56 0000000000000001
[29949.408478]  ffff88068086f718 ffff88068086f708 ffffffff8107dcb7 0000000000000001
[29949.476737]  ffff880856b5e120 0000000000000001 ffff881056dc8000 0000000000004000
[29949.545175] Call Trace:
[29949.613042]  [<ffffffff8162dd56>] dump_stack+0x45/0x57
[29949.680804]  [<ffffffff8107dcb7>] warn_slowpath_common+0x97/0xe0
[29949.748815]  [<ffffffff8107ddb6>] warn_slowpath_fmt+0x46/0x50
[29949.816784]  [<ffffffffa01e0611>] btrfs_alloc_tree_block+0x431/0x4c0 [btrfs]
[29949.884522]  [<ffffffffa02177d9>] ? read_extent_buffer+0xb9/0x120 [btrfs]
[29949.952906]  [<ffffffffa01ca2fc>] __btrfs_cow_block+0x11c/0x540 [btrfs]
[29950.021243]  [<ffffffffa01edc25>] ? btrfs_buffer_uptodate+0x65/0x80 [btrfs]
[29950.089058]  [<ffffffffa01ca8ea>] btrfs_cow_block+0x12a/0x1c0 [btrfs]
[29950.157936]  [<ffffffffa01ce800>] btrfs_search_slot+0x1d0/0xa10 [btrfs]
[29950.225553]  [<ffffffffa01d5032>] lookup_inline_extent_backref+0x122/0x5e0 [btrfs]
[29950.294614]  [<ffffffffa01ee0e5>] ? btrfs_mark_buffer_dirty+0x95/0xd0 [btrfs]
[29950.362736]  [<ffffffffa01d80dd>] __btrfs_free_extent+0x14d/0xc00 [btrfs]
[29950.431597]  [<ffffffff8162be43>] ? __slab_free+0x102/0x200
[29950.500015]  [<ffffffffa01df210>] __btrfs_run_delayed_refs+0x3f0/0x1270 [btrfs]
[29950.570184]  [<ffffffff810b84b6>] ? enqueue_task_fair+0x3d6/0x6e0
[29950.639537]  [<ffffffffa01e2434>] btrfs_run_delayed_refs+0x74/0x280 [btrfs]
[29950.710433]  [<ffffffffa01e28e7>] delayed_ref_async_start+0x37/0x90 [btrfs]
[29950.780266]  [<ffffffffa0224d02>] normal_work_helper+0xc2/0x2a0 [btrfs]
[29950.850726]  [<ffffffffa0225112>] btrfs_extent_refs_helper+0x12/0x20 [btrfs]
[29950.920587]  [<ffffffff810955af>] process_one_work+0x14f/0x410
[29950.990719]  [<ffffffff810959c9>] worker_thread+0x119/0x510
[29951.058965]  [<ffffffff8162fc21>] ? __schedule+0x4f1/0x99f
[29951.126978]  [<ffffffff810958b0>] ? process_scheduled_works+0x40/0x40
[29951.194704]  [<ffffffff8109c119>] kthread+0xc9/0xe0
[29951.262578]  [<ffffffff8109c050>] ? kthread_create_on_node+0x1a0/0x1a0
[29951.331304]  [<ffffffff816347e2>] ret_from_fork+0x42/0x70
[29951.399950]  [<ffffffff8109c050>] ? kthread_create_on_node+0x1a0/0x1a0
[29951.467320] ---[ end trace 487b9b7d8b3ac3b6 ]---

Stefan