System panicked due to hung_task_panic set and huge backlog of NFS writes

Issue

System panicked with this message:

INFO: task bptm:24197 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
bptm          D ffff810c4e0c3a20     0 24197  23213                     (L-TLB)
 ffff8101c34b3d28 0000000000000046 0000000000000000 ffffffff887809f8
 0000000000000050 0000000000000009 ffff81095ac0a830 ffff81103fd4f040
 000001afa4e04f8d 00000000005b3f93 ffff81095ac0aa18 00000016c34b3d88
Call Trace:
 [<ffffffff887809f8>] :sunrpc:xprt_reserve+0x14d/0x161
 [<ffffffff8009b6bc>] recalc_sigpending+0xe/0x25
 [<ffffffff8006edac>] do_gettimeofday+0x40/0x90
 [<ffffffff887df8d1>] :nfs:nfs_wait_bit_uninterruptible+0x0/0xd
 [<ffffffff800637de>] io_schedule+0x3f/0x67
 [<ffffffff887df8da>] :nfs:nfs_wait_bit_uninterruptible+0x9/0xd
 [<ffffffff80063a0a>] __wait_on_bit+0x40/0x6e
 [<ffffffff887df8d1>] :nfs:nfs_wait_bit_uninterruptible+0x0/0xd
 [<ffffffff80063aa4>] out_of_line_wait_on_bit+0x6c/0x78
 [<ffffffff800a3fd0>] wake_bit_function+0x0/0x23
 [<ffffffff887e4648>] :nfs:nfs_sync_inode_wait+0xeb/0x2df
 [<ffffffff887d9ee9>] :nfs:nfs_do_fsync+0x22/0x42
 [<ffffffff800241da>] filp_close+0x36/0x64
 [<ffffffff80039334>] put_files_struct+0x63/0xae
 [<ffffffff80015941>] do_exit+0x352/0x931
 [<ffffffff80049524>] cpuset_exit+0x0/0x88
 [<ffffffff8005d116>] system_call+0x7e/0x83

Kernel panic - not syncing: hung_task: blocked tasks