A different lockdep chain, from test btrfs/014:
[ 229.850151] ======================================================
[ 229.850788] [ INFO: possible circular locking dependency detected ]
[ 229.850788] 3.15.0-rc7-default+ #146 Tainted: G W
[ 229.850788] -------------------------------------------------------
[ 229.850788] btrfs/26558 is trying to acquire lock:
[ 229.850788] (s_active#149){++++.+}, at: [<ffffffff811ee787>]
kernfs_remove+0x27/0x40
[ 229.850788]
[ 229.850788] but task is already holding lock:
[ 229.850788] (&found->groups_sem){++++..}, at: [<ffffffffa0023abe>]
btrfs_remove_block_group+0x24e/0x580 [btrfs]
[ 229.850788]
[ 229.850788] which lock already depends on the new lock.
[ 229.850788]
[ 229.850788]
[ 229.850788] the existing dependency chain (in reverse order) is:
[ 229.850788]
-> #1 (&found->groups_sem){++++..}:
[ 229.850788] [<ffffffff810b0662>] lock_acquire+0x92/0x120
[ 229.850788] [<ffffffff81a0bedc>] down_read+0x4c/0xa0
[ 229.850788] [<ffffffffa004babe>] raid_bytes_show+0x3e/0xd0 [btrfs]
[ 229.850788] [<ffffffff813b9ef6>] kobj_attr_show+0x16/0x20
[ 229.850788] [<ffffffff811f04e9>] sysfs_kf_seq_show+0xd9/0x230
[ 229.850788] [<ffffffff811eec56>] kernfs_seq_show+0x26/0x30
[ 229.850788] [<ffffffff8119ec8f>] seq_read+0xef/0x410
[ 229.850788] [<ffffffff811efa35>] kernfs_fop_read+0x125/0x180
[ 229.850788] [<ffffffff81179fe4>] vfs_read+0xb4/0x180
[ 229.850788] [<ffffffff8117a269>] SyS_read+0x59/0xd0
[ 229.850788] [<ffffffff81a16cd2>] system_call_fastpath+0x16/0x1b
[ 229.850788]
-> #0 (s_active#149){++++.+}:
[ 229.850788] [<ffffffff810afc5c>] __lock_acquire+0x1c4c/0x1fa0
[ 229.850788] [<ffffffff810b0662>] lock_acquire+0x92/0x120
[ 229.850788] [<ffffffff811edc07>] __kernfs_remove+0x2b7/0x380
[ 229.850788] [<ffffffff811ee787>] kernfs_remove+0x27/0x40
[ 229.850788] [<ffffffff811f0b3a>] sysfs_remove_dir+0x5a/0x90
[ 229.850788] [<ffffffff813ba028>] kobject_del+0x18/0x90
[ 229.850788] [<ffffffffa0023cb2>] btrfs_remove_block_group+0x442/0x580
[btrfs]
[ 229.850788] [<ffffffffa005caf4>] btrfs_relocate_chunk+0x624/0x770
[btrfs]
[ 229.850788] [<ffffffffa005f652>] btrfs_balance+0x902/0xf50 [btrfs]
[ 229.850788] [<ffffffffa006b090>] btrfs_ioctl_balance+0x1e0/0x350
[btrfs]
[ 229.850788] [<ffffffffa006d4a9>] btrfs_ioctl+0xc39/0x1830 [btrfs]
[ 229.850788] [<ffffffff8118c381>] do_vfs_ioctl+0x91/0x560
[ 229.850788] [<ffffffff8118c8a3>] SyS_ioctl+0x53/0x80
[ 229.850788] [<ffffffff81a16cd2>] system_call_fastpath+0x16/0x1b
[ 229.850788]
[ 229.850788] other info that might help us debug this:
[ 229.850788]
[ 229.850788] Possible unsafe locking scenario:
[ 229.850788]
[ 229.850788] CPU0 CPU1
[ 229.850788] ---- ----
[ 229.850788] lock(&found->groups_sem);
[ 229.850788] lock(s_active#149);
[ 229.850788] lock(&found->groups_sem);
[ 229.850788] lock(s_active#149);
[ 229.850788]
[ 229.850788] *** DEADLOCK ***
[ 229.850788]
[ 229.850788] 5 locks held by btrfs/26558:
[ 229.850788] #0: (sb_writers#11){.+.+.+}, at: [<ffffffff8119a828>]
mnt_want_write_file+0x28/0x60
[ 229.850788] #1: (&fs_info->volume_mutex){+.+.+.}, at: [<ffffffffa006afe1>]
btrfs_ioctl_balance+0x131/0x350 [btrfs]
[ 229.850788] #2: (sb_internal){.+.+..}, at: [<ffffffffa0035436>]
start_transaction+0x456/0x550 [btrfs]
[ 229.850788] #3: (&fs_info->chunk_mutex){+.+...}, at: [<ffffffffa005927e>]
lock_chunks+0x1e/0x20 [btrfs]
[ 229.850788] #4: (&found->groups_sem){++++..}, at: [<ffffffffa0023abe>]
btrfs_remove_block_group+0x24e/0x580 [btrfs]
[ 229.850788]
[ 229.850788] stack backtrace:
[ 229.850788] CPU: 1 PID: 26558 Comm: btrfs Tainted: G W
3.15.0-rc7-default+ #146
[ 229.850788] Hardware name: Intel Corporation Santa Rosa platform/Matanzas,
BIOS TSRSCRB1.86C.0047.B00.0610170821 10/17/06
[ 229.850788] ffffffff8281faf0 ffff8800652a1818 ffffffff81a07898
0000000000000001
[ 229.850788] ffffffff82827310 ffff8800652a1868 ffffffff810acb74
00000000001d4500
[ 229.850788] ffff8800652a18e8 0000000000000004 ffff880075e0ac78
0000000000000004
[ 229.850788] Call Trace:
[ 229.850788] [<ffffffff81a07898>] dump_stack+0x51/0x71
[ 229.850788] [<ffffffff810acb74>] print_circular_bug+0x214/0x310
[ 229.850788] [<ffffffff810afc5c>] __lock_acquire+0x1c4c/0x1fa0
[ 229.850788] [<ffffffff811ee787>] ? kernfs_remove+0x27/0x40
[ 229.850788] [<ffffffff810b0662>] lock_acquire+0x92/0x120
[ 229.850788] [<ffffffff811ee787>] ? kernfs_remove+0x27/0x40
[ 229.850788] [<ffffffff81a0bb39>] ? mutex_unlock+0x9/0x10
[ 229.850788] [<ffffffff811edc07>] __kernfs_remove+0x2b7/0x380
[ 229.850788] [<ffffffff811ee787>] ? kernfs_remove+0x27/0x40
[ 229.850788] [<ffffffff811ee77f>] ? kernfs_remove+0x1f/0x40
[ 229.850788] [<ffffffff811ee787>] kernfs_remove+0x27/0x40
[ 229.850788] [<ffffffff811f0b3a>] sysfs_remove_dir+0x5a/0x90
[ 229.850788] [<ffffffff813ba028>] kobject_del+0x18/0x90
[ 229.850788] [<ffffffffa0023cb2>] btrfs_remove_block_group+0x442/0x580
[btrfs]
[ 229.850788] [<ffffffff810b112d>] ? trace_hardirqs_on_caller+0x11d/0x1e0
[ 229.850788] [<ffffffffa005caf4>] btrfs_relocate_chunk+0x624/0x770 [btrfs]
[ 229.850788] [<ffffffffa0056ac6>] ? release_extent_buffer+0x36/0xe0 [btrfs]
[ 229.850788] [<ffffffffa005706a>] ? free_extent_buffer+0x4a/0xc0 [btrfs]
[ 229.850788] [<ffffffffa0057080>] ? free_extent_buffer+0x60/0xc0 [btrfs]
[ 229.850788] [<ffffffffa005f652>] btrfs_balance+0x902/0xf50 [btrfs]
[ 229.850788] [<ffffffffa006b090>] ? btrfs_ioctl_balance+0x1e0/0x350 [btrfs]
[ 229.850788] [<ffffffff8116e3fa>] ? kmem_cache_alloc_trace+0x12a/0x180
[ 229.850788] [<ffffffffa006b090>] btrfs_ioctl_balance+0x1e0/0x350 [btrfs]
[ 229.850788] [<ffffffffa006c870>] ? update_ioctl_balance_args+0x130/0x130
[btrfs]
[ 229.850788] [<ffffffffa006d4a9>] btrfs_ioctl+0xc39/0x1830 [btrfs]
[ 229.850788] [<ffffffff8118c381>] do_vfs_ioctl+0x91/0x560
[ 229.850788] [<ffffffff81197959>] ? __fget_light+0x9/0x70
[ 229.850788] [<ffffffff811979c9>] ? __fdget+0x9/0x20
[ 229.850788] [<ffffffff8118c8a3>] SyS_ioctl+0x53/0x80
[ 229.850788] [<ffffffff81a16cd2>] system_call_fastpath+0x16/0x1b
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