RHEL6: system hang with one process holding s_umount and waiting for unfreeze in __sb_start_write, prevents unfreeze due to thaw_bdev needing s_umount

Issue

• Hang while doing a backup / snapshot procedure which freezes / unfreezes filesystems, with symptoms similar to what is described in RHEL6: Deadlock on frozen ext4 filesystem, one process stuck in thaw_bdev waiting on semaphore held by flush thread doing writeback and stuck in start_this_handle, but on a kernel later than kernel-2.6.32-358.el6
• System hang due to a process holding the s_umount semaphore and getting stuck in __sb_start_write, which will prevent any other process from completing thaw_bdev. This process may be a variety of things, such as kswapd, a process doing a sync, or a process doing drop_caches.
• Here is an example of a kswapd backtrace:

PID: 40     TASK: ffff880079f08ae0  CPU: 1   COMMAND: "kswapd0"
 #0 [ffff880079f0d970] schedule at ffffffff81529990
 #1 [ffff880079f0da48] __sb_start_write at ffffffff811900ec
 #2 [ffff880079f0dad8] ext4_delete_inode at ffffffffa00a1ef4 [ext4]
 #3 [ffff880079f0daf8] generic_delete_inode at ffffffff811ac07e
 #4 [ffff880079f0db28] generic_drop_inode at ffffffff811ac1d5
 #5 [ffff880079f0db48] iput at ffffffff811ab022
 #6 [ffff880079f0db68] dentry_iput at ffffffff811a7c10
 #7 [ffff880079f0db88] d_kill at ffffffff811a7d71
 #8 [ffff880079f0dba8] __shrink_dcache_sb at ffffffff811a8106
 #9 [ffff880079f0dc48] shrink_dcache_memory at ffffffff811a8289
#10 [ffff880079f0dca8] shrink_slab at ffffffff8113d4ba
#11 [ffff880079f0dd08] balance_pgdat at ffffffff8114082a
#12 [ffff880079f0de28] kswapd at ffffffff81140be4
#13 [ffff880079f0dee8] kthread at ffffffff8109e66e

• Here is an example of a 'sync' process backtrace

PID: 23562  TASK: ffff8800253c4040  CPU: 2   COMMAND: "sync_proc"
 #0 [ffff880026a7bc78] schedule at ffffffff81534790
 #1 [ffff880026a7bd50] __sb_start_write at ffffffff8119322c
 #2 [ffff880026a7bde0] ext4_delete_inode at ffffffffa01ddf84 [ext4]
 #3 [ffff880026a7be00] generic_delete_inode at ffffffff811af25e
 #4 [ffff880026a7be30] generic_drop_inode at ffffffff811af3b5
 #5 [ffff880026a7be50] iput at ffffffff811ae202
 #6 [ffff880026a7be70] sync_inodes_sb at ffffffff811bd61c
 #7 [ffff880026a7bf00] __sync_filesystem at ffffffff811c3d02
 #8 [ffff880026a7bf20] sync_filesystem at ffffffff811c3f0b
 #9 [ffff880026a7bf40] sys_syncfs at ffffffff811c3f80
#10 [ffff880026a7bf80] tracesys at ffffffff8100b288 (via system_call)