System crashes when removing large files on 3PAR disk that is mounted with discard option.

Issue

• When hung_task_panic is enabled, system crashes while removing large files on 3PAR disk that is mounted with discard option.

INFO: task rm:24315 blocked for more than 300 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
message.
rm      D 0000000000000001     0 24315  13602 0x00000004
 ffff880818d11d08 0000000000000082 0000000000000000 ffff880813ce4dc8
 ffff880817ec4ae0 ffff88102593dc00 0000000000001000 ffff880822baaac0
 ffff88082a817ab8 ffff880818d11fd8 000000000000fb88 ffff88082a817ab8
Call Trace:
 [<ffffffffa013604a>] start_this_handle+0x25a/0x480 [jbd2]
 [<ffffffff81090857>] ? bit_waitqueue+0x17/0xd0
 [<ffffffff81090990>] ? autoremove_wake_function+0x0/0x40
 [<ffffffffa0136455>] jbd2_journal_start+0xb5/0x100 [jbd2]
 [<ffffffffa017dbd8>] ext4_journal_start_sb+0x58/0x90 [ext4]
 [<ffffffffa016cb7d>] ext4_unlink+0x9d/0x2b0 [ext4]
 [<ffffffff81183d5f>] vfs_unlink+0x9f/0xe0
 [<ffffffff81182aaa>] ? lookup_hash+0x3a/0x50
 [<ffffffff81186293>] do_unlinkat+0x183/0x1c0
 [<ffffffff811b5d0e>] ? dnotify_flush+0x7e/0x140
 [<ffffffff8117389d>] ? filp_close+0x5d/0x90
 [<ffffffff81186432>] sys_unlinkat+0x22/0x40
 [<ffffffff8100b072>] system_call_fastpath+0x16/0x1b
Kernel panic - not syncing: hung_task: blocked tasks

• DISCARDs cause a continuous stream of errors if filesystem is mounted with "-o discard" and also cause mkfs to fail compeletely with the "-E discard" option.