Comment 7 for bug 1454903

I've just seen this with 16.04, together with latest HES 4.8 kernel:

uname -a
Linux server 4.8.0-51-generic #54~16.04.1-Ubuntu SMP Wed Apr 26 16:00:28 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

That happened during a btrfs-scrub:

May 7 02:11:52 server kernel: [17884.471597] [<ffffffffc04cbc73>] wait_current_trans.isra.21+0xd3/0x120 [btrfs]
May 7 02:11:52 server kernel: [17884.471617] [<ffffffffc04ce43f>] start_transaction+0x27f/0x4c0 [btrfs]
May 7 02:11:52 server kernel: [17884.471635] [<ffffffffc04ce6d7>] btrfs_join_transaction+0x17/0x20 [btrfs]
May 7 02:11:52 server kernel: [17884.471654] [<ffffffffc04d4b45>] btrfs_dirty_inode+0x35/0xd0 [btrfs]
May 7 02:11:52 server kernel: [17884.471672] [<ffffffffc04da57d>] btrfs_setattr+0x9d/0x330 [btrfs]
May 7 02:11:52 server kernel: [17884.471697] INFO: task btrfs:3559 blocked for more than 120 seconds.
May 7 02:11:52 server kernel: [17884.471851] btrfs D ffff937fa2257b68 0 3559 3518 0x00000000
May 7 02:11:52 server kernel: [17884.471887] [<ffffffffc04cb862>] wait_for_commit.isra.16+0x52/0xa0 [btrfs]
May 7 02:11:52 server kernel: [17884.471909] [<ffffffffc04cc205>] btrfs_wait_for_commit+0x145/0x170 [btrfs]
May 7 02:11:52 server kernel: [17884.471935] [<ffffffffc04bb227>] btrfs_inc_block_group_ro+0x67/0x1b0 [btrfs]
May 7 02:11:52 server kernel: [17884.471961] [<ffffffffc0532d77>] scrub_enumerate_chunks+0x217/0x670 [btrfs]
May 7 02:11:52 server kernel: [17884.471994] [<ffffffffc05344ea>] btrfs_scrub_dev+0x1fa/0x540 [btrfs]
May 7 02:11:52 server kernel: [17884.472019] [<ffffffffc050c25c>] btrfs_ioctl+0x1a7c/0x2010 [btrfs]