Comment 12 for bug 1614887

[ 3988.561634] INFO: task pool:2945 blocked for more than 120 seconds.
[ 3988.561642] Not tainted 4.9.0-040900rc7-generic #201611271731
[ 3988.561643] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 3988.561646] pool D 0 2945 2409 0x00000000
[ 3988.561651] ffffa05cf068a000 ffffa05cb4ac4800 ffffa05ca6851a80 ffffa05cffc19340
[ 3988.561654] ffffa05ca6856a00 ffffc14cc3843ad8 ffffffffb4488453 0000000000000286
[ 3988.561657] 0000000000000038 ffffa05cffc19340 ffffa05cd7db6140 ffffa05ca6851a80
[ 3988.561660] Call Trace:
[ 3988.561670] [<ffffffffb4488453>] ? __schedule+0x233/0x6e0
[ 3988.561673] [<ffffffffb4489130>] ? bit_wait+0x60/0x60
[ 3988.561675] [<ffffffffb4488936>] schedule+0x36/0x80
[ 3988.561677] [<ffffffffb448c02a>] schedule_timeout+0x22a/0x3f0
[ 3988.561781] [<ffffffffc040d933>] ? ulist_add+0x13/0x20 [btrfs]
[ 3988.561785] [<ffffffffb3c63f1e>] ? kvm_clock_get_cycles+0x1e/0x20
[ 3988.561788] [<ffffffffb3cff0a1>] ? ktime_get+0x41/0xb0
[ 3988.561789] [<ffffffffb4489130>] ? bit_wait+0x60/0x60
[ 3988.561790] [<ffffffffb44881b4>] io_schedule_timeout+0xa4/0x110
[ 3988.561791] [<ffffffffb448914b>] bit_wait_io+0x1b/0x60
[ 3988.561793] [<ffffffffb4488cc8>] __wait_on_bit+0x58/0x90
[ 3988.561796] [<ffffffffb3da4cd2>] wait_on_page_bit+0x82/0xa0
[ 3988.561798] [<ffffffffb3cc8eb0>] ? autoremove_wake_function+0x40/0x40
[ 3988.561812] [<ffffffffc03b55af>] prepare_pages+0x19f/0x240 [btrfs]
[ 3988.561826] [<ffffffffc03b5dd9>] __btrfs_buffered_write+0x1f9/0x710 [btrfs]
[ 3988.561840] [<ffffffffc03b9bb0>] btrfs_file_write_iter+0x170/0x4d0 [btrfs]
[ 3988.561843] [<ffffffffb3e369e5>] __vfs_write+0xe5/0x160
[ 3988.561844] [<ffffffffb3e37065>] vfs_write+0xb5/0x1a0
[ 3988.561845] [<ffffffffb3e384c5>] SyS_write+0x55/0xc0
[ 3988.561847] [<ffffffffb448d67b>] entry_SYSCALL_64_fastpath+0x1e/0xad