I've been having some trouble with btrfs. The symptom is that I have higher
than normal wait state but with no tasks using CPU.

Using `echo w > /proc/sysrq-trigger` there is a common theme:


May 08 09:40:44 turnip kernel: task:btrfs-transacti state:D stack:    0 pid: 
841 ppid:     2 flags:0x80004000
May 08 09:40:44 turnip kernel: Call Trace:
May 08 09:40:44 turnip kernel:  __schedule+0x2ff/0x760
May 08 09:40:44 turnip kernel:  ? bit_wait+0x50/0x50
May 08 09:40:44 turnip kernel:  schedule+0x2f/0xa0
May 08 09:40:44 turnip kernel:  io_schedule+0x12/0x40
May 08 09:40:44 turnip kernel:  bit_wait_io+0xd/0x50
May 08 09:40:44 turnip kernel:  __wait_on_bit+0x2c/0x90
May 08 09:40:44 turnip kernel:  out_of_line_wait_on_bit+0x91/0xb0
May 08 09:40:44 turnip kernel:  ? init_wait_var_entry+0x50/0x50
May 08 09:40:44 turnip kernel:  write_all_supers+0x278/0xa00 [btrfs]
May 08 09:40:44 turnip kernel:  ? btrfs_commit_transaction+0x790/0xa60 [btrfs]
May 08 09:40:44 turnip kernel:  btrfs_commit_transaction+0x790/0xa60 [btrfs]
May 08 09:40:44 turnip kernel:  ? start_transaction+0xc5/0x540 [btrfs]
May 08 09:40:44 turnip kernel:  transaction_kthread+0x148/0x160 [btrfs]
May 08 09:40:44 turnip kernel:  ? btrfs_cleanup_transaction+0x5c0/0x5c0 [btrfs]
May 08 09:40:44 turnip kernel:  kthread+0x10d/0x130
May 08 09:40:44 turnip kernel:  ? kthread_park+0xa0/0xa0
May 08 09:40:44 turnip kernel:  ret_from_fork+0x35/0x40



I tried to use `btrfs check` (offline) and it complains of free space
mis-matches, so I zero the v2 free space and then when I try to zero the v1
free space:

turnip:~ # btrfs check  -p --clear-space-cache v1
/dev/disk/by-uuid/ea5898b2-3e9f-4a4b-ae54-5e3e1d13c3ff 
Opening filesystem to check...
Checking filesystem on /dev/disk/by-uuid/ea5898b2-3e9f-4a4b-ae54-5e3e1d13c3ff
UUID: ea5898b2-3e9f-4a4b-ae54-5e3e1d13c3ff
Failed to find [401685020672, 168, 16384]
btrfs unable to find ref byte nr 401685037056 parent 0 root 2  owner 0 offset 0
transaction.c:195: btrfs_commit_transaction: BUG_ON `ret` triggered, value -5
btrfs(+0x51f99)[0x560c29c5df99]
btrfs(btrfs_commit_transaction+0x1ae)[0x560c29c5e58e]
btrfs(btrfs_clear_free_space_cache+0xdc)[0x560c29c510bc]
btrfs(+0x157fa)[0x560c29c217fa]
btrfs(cmd_check+0x103a)[0x560c29c73cca]
btrfs(main+0x8e)[0x560c29c2b08e]
/lib64/libc.so.6(__libc_start_main+0xef)[0x7feccb3382bd]
btrfs(_start+0x2a)[0x560c29c2b28a]
Aborted (core dumped)
turnip:~ # 


Currently running a basic 'check'.
Kernel is latest. I regularly run scrub/etc. via btrfsmaintenance