我们是运行 Xenial (16.04) 和 Linux 内核 4.15.0-36-generic 的 Ubuntu 商店。我们最近从 Ubuntu 14.04/Linux 内核 4.4.0-23 迁移了我们的应用程序代码。我们在 16.04 环境中遇到了一些系统挂起问题（如下所述），而在 14.04 和 4.4 内核中没有遇到过这些问题。
我们的二级存储（256GB Micron SSD）安装在 /dev/sdb 上，文件系统为 BTRFS。在执行大型文件复制/修改操作时，系统会挂起，系统控制台 tty 上会显示以下消息：
> [147778.018904] INFO: task btrfs-transacti:440 blocked for more than
> 120 seconds. [147778.026992] Tainted: P OE
> 4.15.0-36-generic #1 [147778.033914] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [147778.042918] INFO: task auditd:1138 blocked for more than 120
> seconds. [147778.050218] Tainted: P OE
> 4.15.0-36-generic #1 [147778.057116] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [147778.066125] INFO: task cron:24933 blocked for more than 120
> seconds. [147778.073317] Tainted: P OE
> 4.15.0-36-generic #1 [147778.080225] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [147778.089664] INFO: task cron:24934 blocked for more than 120
> seconds. [147778.096866] Tainted: P OE
> 4.15.0-36-generic #1 [147778.103764] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [147778.113159] INFO: task cron:24935 blocked for more than 120
> seconds. [147778.120360] Tainted: P OE
> 4.15.0-36-generic #1 [147778.127267] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [147898.850483] INFO: task btrfs-transacti:440 blocked for more than
> 120 seconds. [147898.858569] Tainted: P OE
> 4.15.0-36-generic #1 [147898.865490] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
这种情况有时会在关机（重启）或系统启动期间发生。在此期间，我们在后台确实有几个文件操作正在进行。我们能够从仍处于活动状态的 ssh 会话中收集“dmesg”输出：
[ 1034.371130] perf: interrupt took too long (14517 > 14503), lowering kernel.perf_event_max_sample_rate to 13750
[ 1157.254266] BTRFS info (device sdb3): disk space caching is enabled
[ 1157.290278] BTRFS info (device sdb3): disk space caching is enabled
[ 1157.312982] BTRFS info (device sdb3): disk space caching is enabled
[ 1157.332657] BTRFS info (device sdb3): disk space caching is enabled
[ 1248.012861] ata1.00: Enabling discard_zeroes_data
[ 1248.020748] sda: sda1
sda1: <solaris: [s0] sda5 [s2] sda6 [s8] sda7 >
[ 1257.996027] BTRFS info (device sdb3): qgroup scan completed (inconsistency flag cleared)
[ 1451.046237] INFO: task kworker/u24:0:5 blocked for more than 120 seconds.
[ 1451.054217] Tainted: P OE 4.15.0-36-generic #1
[ 1451.061152] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 1451.069963] kworker/u24:0 D 0 5 2 0x80000000
[ 1451.070199] Workqueue: btrfs-endio-write btrfs_endio_write_helper [btrfs]
[ 1451.070257] Call Trace:
[ 1451.070312] __schedule+0x3d6/0x8b0
[ 1451.070332] ? blk_rq_map_sg+0x13c/0x550
[ 1451.070336] schedule+0x36/0x80
[ 1451.070366] btrfs_tree_lock+0xef/0x210 [btrfs]
[ 1451.070381] ? wait_woken+0x80/0x80
[ 1451.070400] btrfs_lock_root_node+0x34/0x50 [btrfs]
[ 1451.070417] btrfs_search_slot+0x914/0x9d0 [btrfs]
[ 1451.070421] ? update_load_avg+0x5e0/0x700
[ 1451.070442] btrfs_lookup_file_extent+0x49/0x60 [btrfs]
[ 1451.070465] __btrfs_drop_extents+0x19e/0xe60 [btrfs]
[ 1451.070488] ? __set_extent_bit+0x466/0x5a0 [btrfs]
[ 1451.070510] ? __set_extent_bit+0x466/0x5a0 [btrfs]
[ 1451.070513] ? _cond_resched+0x1a/0x50
[ 1451.070515] ? _cond_resched+0x1a/0x50
[ 1451.070535] insert_reserved_file_extent.constprop.68+0x90/0x2d0 [btrfs]
[ 1451.070557] ? start_transaction+0x9b/0x440 [btrfs]
[ 1451.070578] btrfs_finish_ordered_io+0x300/0x720 [btrfs]
[ 1451.070601] ? end_compressed_bio_write+0x102/0x140 [btrfs]
[ 1451.070622] finish_ordered_fn+0x15/0x20 [btrfs]
[ 1451.070644] normal_work_helper+0xcb/0x320 [btrfs]
[ 1451.070667] btrfs_endio_write_helper+0x12/0x20 [btrfs]
[ 1451.070672] process_one_work+0x14d/0x410
[ 1451.070675] worker_thread+0x4b/0x460
[ 1451.070679] kthread+0x105/0x140
[ 1451.070680] ? process_one_work+0x410/0x410
[ 1451.070683] ? kthread_destroy_worker+0x50/0x50
[ 1451.070686] ret_from_fork+0x35/0x40
[ 1451.070733] INFO: task kworker/u24:1:101 blocked for more than 120 seconds.
[ 1451.078559] Tainted: P OE 4.15.0-36-generic #1
[ 1451.085426] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 1451.094228] kworker/u24:1 D 0 101 2 0x80000000
[ 1451.094275] Workqueue: btrfs-endio-write btrfs_endio_write_helper [btrfs]
[ 1451.094278] Call Trace:
[ 1451.094282] __schedule+0x3d6/0x8b0
[ 1451.094285] schedule+0x36/0x80
[ 1451.094307] btrfs_tree_lock+0xef/0x210 [btrfs]
[ 1451.094310] ? wait_woken+0x80/0x80
[ 1451.094327] btrfs_lock_root_node+0x34/0x50 [btrfs]
[ 1451.094343] btrfs_search_slot+0x914/0x9d0 [btrfs]
[ 1451.094363] btrfs_lookup_file_extent+0x49/0x60 [btrfs]
[ 1451.094384] __btrfs_drop_extents+0x19e/0xe60 [btrfs]
[ 1451.094407] ? __set_extent_bit+0x466/0x5a0 [btrfs]
[ 1451.094428] ? __set_extent_bit+0x466/0x5a0 [btrfs]
[ 1451.094431] ? _cond_resched+0x1a/0x50
[ 1451.094434] ? _cond_resched+0x1a/0x50
[ 1451.094455] insert_reserved_file_extent.constprop.68+0x90/0x2d0 [btrfs]
[ 1451.094475] ? start_transaction+0x9b/0x440 [btrfs]
[ 1451.094495] btrfs_finish_ordered_io+0x300/0x720 [btrfs]
[ 1451.094499] ? __switch_to_asm+0x40/0x70
[ 1451.094503] ? __switch_to_asm+0x34/0x70
[ 1451.094505] ? __switch_to_asm+0x40/0x70
[ 1451.094524] finish_ordered_fn+0x15/0x20 [btrfs]
[ 1451.094546] normal_work_helper+0xcb/0x320 [btrfs]
[ 1451.094569] btrfs_endio_write_helper+0x12/0x20 [btrfs]
[ 1451.094571] process_one_work+0x14d/0x410
[ 1451.094575] worker_thread+0x4b/0x460
[ 1451.094578] kthread+0x105/0x140
[ 1451.094580] ? process_one_work+0x410/0x410
[ 1451.094582] ? kthread_destroy_worker+0x50/0x50
[ 1451.094585] ret_from_fork+0x35/0x40
[ 1451.094603] INFO: task kworker/u24:2:271 blocked for more than 120 seconds.
[ 1451.102453] Tainted: P OE 4.15.0-36-generic #1
[ 1451.109330] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 1451.118138] kworker/u24:2 D 0 271 2 0x80000000
[ 1451.118165] Workqueue: btrfs-endio-write btrfs_endio_write_helper [btrfs]
[ 1451.118173] Call Trace:
[ 1451.118180] __schedule+0x3d6/0x8b0
[ 1451.118183] schedule+0x36/0x80
[ 1451.118205] btrfs_tree_lock+0xef/0x210 [btrfs]
[ 1451.118215] ? wait_woken+0x80/0x80
[ 1451.118231] btrfs_lock_root_node+0x34/0x50 [btrfs]
[ 1451.118252] btrfs_search_slot+0x914/0x9d0 [btrfs]
[ 1451.118273] btrfs_lookup_file_extent+0x49/0x60 [btrfs]
[ 1451.118296] __btrfs_drop_extents+0x19e/0xe60 [btrfs]
[ 1451.118319] ? __set_extent_bit+0x466/0x5a0 [btrfs]
[ 1451.118342] ? __set_extent_bit+0x466/0x5a0 [btrfs]
[ 1451.118346] ? _cond_resched+0x1a/0x50
[ 1451.118368] insert_reserved_file_extent.constprop.68+0x90/0x2d0 [btrfs]
[ 1451.118389] ? start_transaction+0x9b/0x440 [btrfs]
[ 1451.118412] btrfs_finish_ordered_io+0x300/0x720 [btrfs]
[ 1451.118435] finish_ordered_fn+0x15/0x20 [btrfs]
[ 1451.118459] normal_work_helper+0xcb/0x320 [btrfs]
[ 1451.118484] btrfs_endio_write_helper+0x12/0x20 [btrfs]
[ 1451.118487] process_one_work+0x14d/0x410
[ 1451.118490] worker_thread+0x4b/0x460
[ 1451.118493] kthread+0x105/0x140
[ 1451.118495] ? process_one_work+0x410/0x410
[ 1451.118498] ? kthread_destroy_worker+0x50/0x50
[ 1451.118501] ret_from_fork+0x35/0x40
以下是我们的 /etc/fstab 条目，显示我们已经逐一尝试过的各种 BTRFS 挂载选项：
UUID=3a899529-c5e4-42ae-a4ea-7b645f9bcb59 / btrfs defaults,autodefrag,thread_pool=4,compress=lzo,noatime,subvol=@/netvisor-1 0 1
UUID=3a899529-c5e4-42ae-a4ea-7b645f9bcb59 /home btrfs defaults,autodefrag,thread_pool=4,compress=lzo,noatime,subvol=@home 0 2
UUID=3a899529-c5e4-42ae-a4ea-7b645f9bcb59 /var/nvOS/log btrfs defaults,autodefrag,thread_pool=4,compress=lzo,noatime,subvol=@var_nvOS_log 0 2
UUID=3a899529-c5e4-42ae-a4ea-7b645f9bcb59 /.rootbe btrfs defaults,autodefrag,thread_pool=4,compress=lzo,noatime,subvol=@ 0 1
这是否表明我们遇到了一些已知的 BTRFS 文件系统死锁问题，而这些问题可能已在后续的内核版本中得到修复？如果是这样，我们应该如何在当前内核中获取这些修复？目前，升级内核对我们来说不是一个选择。是否有任何其他 BTRFS 挂载选项或任何其他系统可调参数可以尝试规避此挂起问题。提前致谢。