Btrfs devel - Aug 2014 - Stack dumps in use_block_rsv while rebalancing ("block rsv returned -28")

I already posted this in the thread "ENOSPC with mkdir and rename",
but now I have a device with 100GB unallocated on the "btrfs fi sh"
output, and when I run a rebalance of the form:
> btrfs filesystem balance start -dusage=50 -musage=10 "$mount"
I get more than 75 of such stack traces contaminating the klog. I've
put some of them up in a gist here:
https://gist.github.com/pwaller/1df8a7efc2f10343f2e3 and one of them
is reproduced below.

Is this harmful or expected? Are there any workarounds?

Thanks,

- Peter

[376007.681938] ------------[ cut here ]------------
[376007.681957] WARNING: CPU: 1 PID: 27021 at
/home/apw/COD/linux/fs/btrfs/
extent-tree.c:6946
use_block_rsv+0xfd/0x1a0 [btrfs]()
[376007.681958] BTRFS: block rsv returned -28
[376007.681959] Modules linked in: softdog tcp_diag inet_diag dm_crypt
ppdev xen_fbfront fb_sys_fops syscopyarea sysfillrect sysimgblt
i2c_piix4 serio_raw parport_pc parport mac_hid isofs xt_tcpudp
iptable_filter xt_owner ip_tables x_tables btrfs xor raid6_pq
crct10dif_pclmul crc32_pclmul ghash_clmulni_intel aesni_intel
aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd floppy psmouse
[376007.681980] CPU: 1 PID: 27021 Comm: pam_script_ses_ Tainted: G
   W     3.15.7-031507-generic #201407281235
[376007.681981] Hardware name: Xen HVM domU, BIOS 4.2.amazon 05/23/2014
[376007.681983]  0000000000001b22 ffff8800acca39d8 ffffffff8176f115
0000000000000007
[376007.681986]  ffff8800acca3a28 ffff8800acca3a18 ffffffff8106ceac
ffff8801efc37870
[376007.681989]  ffff88017db0ff00 ffff8801aedcd800 0000000000001000
ffff88001c987000
[376007.681992] Call Trace:
[376007.682000]  [<ffffffff8176f115>] dump_stack+0x46/0x58
[376007.682005]  [<ffffffff8106ceac>] warn_slowpath_common+0x8c/0xc0
[376007.682008]  [<ffffffff8106cf96>] warn_slowpath_fmt+0x46/0x50
[376007.682016]  [<ffffffffa00d9d1d>] use_block_rsv+0xfd/0x1a0 [btrfs]
[376007.682024]  [<ffffffffa00de687>] btrfs_alloc_free_block+0x57/0x220
[btrfs]
[376007.682027]  [<ffffffff8178033c>] ? __do_page_fault+0x28c/0x550
[376007.682031]  [<ffffffff8119749f>] ? page_add_file_rmap+0x6f/0xb0
[376007.682037]  [<ffffffffa00c8a3c>] btrfs_copy_root+0xfc/0x2b0 [btrfs]
[376007.682041]  [<ffffffff811c60b9>] ? memcg_check_events+0x29/0x50
[376007.682051]  [<ffffffffa013a583>] ? create_reloc_root+0x33/0x2c0
[btrfs]
[376007.682061]  [<ffffffffa013a743>] create_reloc_root+0x1f3/0x2c0
[btrfs]
[376007.682064]  [<ffffffff811dd073>] ? generic_permission+0xf3/0x120
[376007.682073]  [<ffffffffa0140eb8>] btrfs_init_reloc_root+0xb8/0xd0
[btrfs]
[376007.682082]  [<ffffffffa00ee967>]
record_root_in_trans.part.30+0x97/0x100 [btrfs]
[376007.682090]  [<ffffffffa00ee9f4>] record_root_in_trans+0x24/0x30
[btrfs]
[376007.682098]  [<ffffffffa00efeb1>]
btrfs_record_root_in_trans+0x51/0x80 [btrfs]
[376007.682106]  [<ffffffffa00f13d6>]
start_transaction.part.35+0x86/0x560 [btrfs]
[376007.682109]  [<ffffffff8132c197>] ? apparmor_capable+0x27/0x80
[376007.682117]  [<ffffffffa00f18d9>] start_transaction+0x29/0x30 [btrfs]
[376007.682125]  [<ffffffffa00f19a7>] btrfs_join_transaction+0x17/0x20
[btrfs]
[376007.682133]  [<ffffffffa00f7fa8>] btrfs_dirty_inode+0x58/0xe0 [btrfs]
[376007.682141]  [<ffffffffa00fcaf2>] btrfs_setattr+0xa2/0xf0 [btrfs]
[376007.682144]  [<ffffffff811eec74>] notify_change+0x1c4/0x3b0
[376007.682146]  [<ffffffff811dde96>] ? final_putname+0x26/0x50
[376007.682149]  [<ffffffff811d088d>] chown_common+0x16d/0x1a0
[376007.682153]  [<ffffffff811f2b08>] ? __mnt_want_write+0x58/0x70
[376007.682156]  [<ffffffff811d1a8f>] SyS_fchownat+0xbf/0x100
[376007.682159]  [<ffffffff811d1aed>] SyS_chown+0x1d/0x20
[376007.682163]  [<ffffffff817858bf>] tracesys+0xe1/0xe6
[376007.682165] ---[ end trace 1853311c87a5cd94 ]---
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