Why did my PPC64 machine become unresponsive after a series of hung_task messages?

Issue

• PPC64 machine printed a series of hung_task messages e.g.

"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
flush-253:5   D 0000000000000000     0  1268      2 0x00008000
Call Trace:
[c000000fe83f6760] [c000000fe83f6810] 0xc000000fe83f6810 (unreliable)
[c000000fe83f6930] [c0000000000142c8] .__switch_to+0xf8/0x1d0
[c000000fe83f69c0] [c0000000005aaaf4] .schedule+0x404/0xd70
[c000000fe83f6cc0] [d0000000072e1c7c] .wait_barrier+0xec/0x1c0 [raid1]
[c000000fe83f6da0] [d0000000072e49fc] .make_request+0xac/0x8e0 [raid1]
[c000000fe83f6ee0] [c00000000048625c] .md_make_request+0x11c/0x300
[c000000fe83f6fb0] [c0000000002d3a3c] .generic_make_request+0x36c/0x550
[c000000fe83f70c0] [c0000000002d3cb4] .submit_bio+0x94/0x130
[c000000fe83f7190] [c0000000001fe59c] .submit_bh+0x15c/0x1c0
[c000000fe83f7220] [c0000000002031f8] .__block_write_full_page+0x328/0x560
[c000000fe83f7310] [d00000000925ad58] .ext4_writepage+0x2b8/0x460 [ext4]
[c000000fe83f73c0] [d00000000925a9a0] .mpage_da_submit_io+0x110/0x1d0 [ext4]
[c000000fe83f7510] [d00000000925d124] .write_cache_pages_da+0x224/0x4b0 [ext4]
[c000000fe83f76b0] [d00000000925d6e8] .ext4_da_writepages+0x338/0x6b0 [ext4]
[c000000fe83f7850] [c00000000016b574] .do_writepages+0x44/0x70
[c000000fe83f78c0] [c0000000001f53b4] .writeback_single_inode+0xe4/0x320
[c000000fe83f7970] [c0000000001f5de0] .writeback_sb_inodes+0x130/0x210
[c000000fe83f7a40] [c0000000001f66c4] .writeback_inodes_wb+0x124/0x250
[c000000fe83f7b10] [c0000000001f6afc] .wb_writeback+0x30c/0x430
[c000000fe83f7c80] [c0000000001f6e64] .wb_do_writeback+0x244/0x330
[c000000fe83f7d80] [c0000000001f6fcc] .bdi_writeback_task+0x7c/0x250
[c000000fe83f7e40] [c00000000017ee64] .bdi_start_fn+0xa4/0x150
[c000000fe83f7ed0] [c0000000000bcb6c] .kthread+0xbc/0xd0
[c000000fe83f7f90] [c000000000032034] .kernel_thread+0x54/0x70