RHEL6.3: "INFO: task xfssyncd/dm-0:56457 blocked for more than 120 seconds." is output

Issue

• xfs filesystem locks up and "blocked for more than 120 seconds" messages involving xfs backtraces, in particular xlog_grant_log_space and xlog_wait
• Bulk removal of several hundreds of data files, on the large xfs filesystem ran into a blocked state - all processes starting IO on that fs blocked. A remount (mount -o remount,rw /xfs-mtpt) also blocks. Only reboot escape from that state. Following messages are seen in /var/log/messages.

kernel: INFO: task xfssyncd/dm-0:56457 blocked for more than 120 seconds.
kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kernel: xfssyncd/dm-0 D 0000000000000010     0 56457      2 0x00000080
kernel: ffff880fc5a15d20 0000000000000046 0000000000000000 0000000000acff08
kernel: ffff881febb2c840 ffff882016470c00 0000000000000000 0000000000000000
kernel: ffff880fbb02da78 ffff880fc5a15fd8 000000000000f4e8 ffff880fbb02da78
kernel: Call Trace:
kernel: [<ffffffffa05d61a3>] _xfs_log_force_lsn+0x183/0x290 [xfs]
kernel: [<ffffffff8105e7f0>] ? default_wake_function+0x0/0x20
kernel: [<ffffffffa05e35e5>] _xfs_trans_commit+0x295/0x2a0 [xfs]
kernel: [<ffffffffa05c60c4>] xfs_fs_log_dummy+0x64/0x90 [xfs]
kernel: [<ffffffffa05d39e4>] ? xfs_log_need_covered+0x94/0xd0 [xfs]
kernel: [<ffffffffa05f8821>] xfs_sync_worker+0x81/0x90 [xfs]
kernel: [<ffffffffa05f870e>] xfssyncd+0x17e/0x210 [xfs]
kernel: [<ffffffffa05f8590>] ? xfssyncd+0x0/0x210 [xfs]
kernel: [<ffffffff81090726>] kthread+0x96/0xa0
kernel: [<ffffffff8100c14a>] child_rip+0xa/0x20
kernel: [<ffffffff81090690>] ? kthread+0x0/0xa0
kernel: [<ffffffff8100c140>] ? child_rip+0x0/0x20